mu4e: workspace restriction doesn't apply to mu4e links

**What did you expect to happen?**
I call `=mu4e`, which opens `mu4e` in the `*mu4e*` buffer. When I call `org-capture` on an email, I get an `org` entry with a `mu4e` link to the email. When I click that link, I expect the email to open in the `*mu4e*` workspace.

**What actually happened?**
When I click the `mu4e` link, the email opens in the current window rather than in the `*mu4e*` workspace. This also moves the whole `mu4e` instance to this window. When I press `q` to exit the email, I get the root `mu4e` interface and the `*mu4e*` workspace is gone.

**Steps to reproduce:**
1. `M-x =mu4e`
2. Open your inbox, maybe with `bt`.
3. Capture an email with `SPC X` or `M-x org-capture`.
4. Save the entry.
5. Open a new workspace, open your `todo.org` file.
6. Click the captured email link.

**System information:**

```
SYSTEM type gnu/linux
config x86_64-pc-linux-gnu
shell /run/current-system/sw/bin/fish
uname Linux 5.4.67 #1-NixOS SMP Wed Sep 23 10:40:47 UTC 2020 x86_64
path (~/.netlify/helper/bin ~/.config/emacs/bin ~/.go/bin ~/.cargo/bin ~/.npm/bin ~/bin /run/wrappers/bin ~/.nix-profile/bin /etc/profiles/per-user/$USER/bin /nix/var/nix/profiles/default/bin /run/current-system/sw/bin /nix/store/192g2szhzzw5hi0ph3487sm2f3ci1cca-emacs-gcc-20200924.0/libexec/emacs/28.0.50/x86_64-pc-linux-gnu/)
EMACS dir ~/dotfiles/emacs/.config/emacs/
version 28.0.50
build Sep 26, 2020
buildopts --prefix=/nix/store/192g2szhzzw5hi0ph3487sm2f3ci1cca-emacs-gcc-20200924.0 --disable-build-details --with-modules --with-x-toolkit=gtk3 --with-xft --with-cairo --with-nativecomp CFLAGS=-DMAC_OS_X_VERSION_MAX_ALLOWED=101200
features XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY INOTIFY LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES NATIVE_COMP THREADS LIBSYSTEMD JSON PDUMPER
traits (server-running envvar-file)
DOOM dir ~/dotfiles/emacs/.config/doom/
version 2.0.9
build HEAD -> mu4e-gmail-flag, origin/develop, origin/HEAD, develop 1456108d5 2020-09-10 18:03:32 -0400
elc-files 0
modules (:input chinese :desktop (&user window-manager) :completion (company +childframe) (ivy +childframe +prescient +icons) :ui doom doom-dashboard hl-todo hydra ophints (popup +defaults) ligatures treemacs unicode vc-gutter window-select workspaces :editor (evil +everywhere) file-templates fold (format +onsave) multiple-cursors :emacs (dired +icons +ranger) electric (undo +tree) vc :term vterm :checkers (syntax +childframe) (spell +aspell +everywhere) :tools direnv editorconfig (eval +overlay) lookup lsp (magit +forge) make pdf terraform :lang (cc +lsp) data emacs-lisp (go +lsp) json (javascript +lsp) kotlin (latex +lsp) lua markdown nix (org +dragndrop +pomodoro) php (python +lsp) racket rest (rust +lsp) (sh +fish +lsp) (swift +lsp) (web +lsp) yaml :email mu4e :app calendar :config (default +bindings +smartparens))
packages ((tree-sitter :recipe (:host github :repo ubolonton/emacs-tree-sitter :files (lisp/*.el src Cargo.toml Cargo.lock))) (tree-sitter-langs :recipe (:host github :repo ubolonton/emacs-tree-sitter :files (langs/*.el langs/queries))) (emojify) (theme-changer :pin 61945695a30d678e6a5d47cbe7c8aff59a8c30ea) (ewal) (ewal-doom-themes) (graphql-mode) (polymode) (poly-markdown) (poly-org) (literate-calc-mode) (string-inflection) (evil-owl) (flycheck-inline :recipe (:host github :repo loafofpiecrust/flycheck-inline :branch moving-point)) (flycheck-posframe :disable t) (flycheck-popup-tip :disable t) (org-msg :disable t) (mu4e-alert) (mu4e-send-delay :recipe (:host github :repo cacology/mu4e-send-delay)) (olivetti) (org-ref) (citeproc-org) (transmission :disable t) (exwm-edit :recipe (:no-native-compile t)) (desktop-environment) (mini-modeline) (anzu) (evil-anzu) (mini-frame :disable t) (mount-mode :recipe (:host github :repo zellerin/mount-mode)) (ivy-posframe :recipe (:no-native-compile t)) (ivy-avy) (org-cv :recipe (:host gitlab :repo Titan-C/org-cv)) (fcitx :disable t) (bitwarden :recipe (:host github :repo seanfarley/emacs-bitwarden)) (zoom) (vterm :recipe (:no-native-compile t)))
unpin (n/a)
elpa (vterm use-package ivy-fuz ivy fuz bind-key)
```