Add Google OAuth support

[ChristophWurst]

Clean version of #6830. Reviewable commit by commit.

Fixes #6454

To do

• Google Oauth admin settings
• Account setup flow adaptation
  • Requires Add guest content component nextcloud-libraries/nextcloud-vue#3369
• Token refresh logic
• Account re-auth for expired/revoke refresh tokens
  • Fix handling of account update API response #7476
• (App) password to oauth migration
• Edge case: email mismatch between Mail form and Google consent flow. Fetch email from Gmail so the token definitely works. Consent windows allows us to "hint" the account email address
• Documentation

How to test

Scenario one: fresh avocado account

1. Follow the admin docs to obtain and save a client ID and secret
2. Open the app
3. Enter your gmail address

• See the form submit button change to Sign in with Google

4. Follow the instructions on the screen

Scenario two: account migration

0. Unlink gmail oauth if you set a client ID/secret already
1. Enable 2FA for your gmail account, create an app password
2. Open the app
3. Add the account using the app password - the form submit button should say Connect
4. Follow the admin docs to obtain and save a client ID and secret
5. Open the app - account should still sync and open mail
6. Open account settings, scroll to server settings - the form submit button should say 'Reconnect Google account*
7. Reconnect the account - it now uses oauth instead of the app password

Screenshots

Screen	Shot
Admin settings
Before setup
During setup

[ChristophWurst]

https://github.com/nextcloud/nextcloud-vue/releases/tag/v7.1.0-beta.0 has arrived

[miaulalala]

WHat I also found was when adding an account, removing it and then adding it again, the signup process isn't smooth any more. i don't know why that is but I had to do the login twice any time the app is already authorised with Gmail to get to a logged in state

[ChristophWurst]

I had to do the login twice any time the app is already authorised with Gmail to get to a logged in state

The login into your Google account?

[miaulalala]

I had to do the login twice any time the app is already authorised with Gmail to get to a logged in state

The login into your Google account?

yes. So the flow would be:

• enter gmail address
• get oauth popup
• accept the popup
• popup closes but login form is displayed with the popup closed text
• do the whole thing again
• finally arrive at the logged in mail account page

[ChristophWurst]

popup closes but login form is displayed with the popup closed text

When my popup closes I see

and then

No double login.

[ChristophWurst]

Alright. This works. Could I get a review and approval @GretaD @miaulalala? :)

[miaulalala]

Should we mention app passwords as well? Or are they not going to work any longer?

[ChristophWurst]

They work, but only if you use 2FA.

[miaulalala]

Looks good to me, tested and works

[GretaD]

looks good