Checking email invite account is unclear when no IS set

Cloned from https://github.com/vector-im/riot-web/issues/10669

Since it's now possible to be disconnected from an identity server entirely (your session has no active identity server), it's unclear how some IS dependent features should function, such as:

Checking an email invite in the invite preview bar to ensure the invited email is for the active account (in case you have multiple Matrix accounts in different tabs, etc.)

I think we are at a stage where @lampholder needs to work out how the product should function for this case.

After some discussion, we'd like to use the following text for the different combinations:

1. Case: Email not added to account
   Title: This invite to %(roomName)s was sent to somebody@example.com which is not associated with your account.
   Subtitle: Link this email with your account in Settings to receive invites directly in Riot.

2. Case: Email added to account, no ID server
   Title: This invite to %(roomName)s was sent to somebody@example.com.
   Subtitle: Use an identity server in Settings to receive invites directly in Riot.

3. Case: Email added to account, have ID server but email not bound
   Title: This invite to %(roomName)s was sent to somebody@example.com.
   Subtitle: Share this email in Settings to receive invites directly in Riot.

4. Case: Email added to account, email bound on ID server (success case, unchanged from current)
   Do you want to join %(roomName)s?
   userName invited you

In all cases, the button will just say "Join the discussion".

Start by tracing the invite preview bar's email/account check and how it handles sessions without an identity server. Compare the four cases and the specified title, subtitle, and button text; done means each combination displays the agreed wording and preserves the unchanged success case.