Allow selection of multiple tags

Often, I use multiple tags for an email, which now means I have to hit the shortcut and select a tag one by one. It would be nice to be able to select multiple tags at a time.

No files or tests are named. Start by locating the shortcut handler and the current one-tag selection flow, then clarify how multiple tags should be chosen and applied in one invocation. Done means a user can select multiple tags for an email without repeating the shortcut.

Written by the indexing model from the issue text.