Can not sign in HackMD in PWA

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### Read our Official tutorial

It's on https://hackmd.io/c/tutorials/

或是中文版 https://hackmd.io/c/tutorials-tw/

### What's the problem
In PWA I can't sign in HackMD or select a note on iOS.

#### Current behavior

Provide steps to reproduce:

1. Open 'Overview' page on iOS Safari Browser
2. 'Add to Home Screen'
3. Tap 'HackMD app' in Home Screen
4. Opened HackMD Top Page in PWA
5. Tap 'Sign in' button
6. Switch to Safari Browser

or

1. Open 'note' page on iOS Safari Browser
2. 'Add to Home Screen'
3. Tap 'HackMD app' in Home Screen
4. Opened the note in PWA but guest mode
5. Sign in with Email+Password
6. Can edit the note
7. Tap 'HackMD Logo' to select other notes
8. Switch to Safari Browser

#### Expected behavior

1. Open 'Overview' page on iOS Safari Browser
2. 'Add to Home Screen'
3. Tap 'HackMD app' in Home Screen
4. Opened HackMD Top Page in PWA
5. Tap 'Sign in' button
6. **Open sign in page in PWA**

or

1. Open 'note' page on iOS Safari Browser
2. 'Add to Home Screen'
3. Tap 'HackMD app' in Home Screen
4. Opened the note in PWA but guest mode
5. Sign in with Email+Password
6. Can edit the note
7. Tap 'HackMD Logo' to select other notes
8. **Open 'My Workspace' in PWA**

#### Environment

I use HackMD on:

**Mobile**

- Device: [iPhone XS Max, iPad Pro 10.5]
- OS: [iOS 13.3, iPadOS 13.3]
- Browser: [safari]
- Version: [604.1]

#### Additional context

Although this may be not critical bug, I'm happy if i could use PWA HackMD!

Beginne damit, die gemeldeten Abläufe in iOS Safari und der HackMD PWA zu reproduzieren: Melde dich auf der Overview-Seite an, melde dich dann in einer Notiz an und wähle das HackMD-Logo aus. Als erledigt gilt die Aufgabe, wenn die Anmeldung innerhalb der PWA geöffnet wird und das Logo dort My Workspace öffnet, anstatt zu Safari zu wechseln.

Vom Indexierungsmodell aus dem Issue-Text verfasst.