Submitting (forbidden) tags through collaborate

### Expected
Collaborate email input should not accept entries like: `hi @ there!`, ` @ `, `I'm a usual tag @` etc

### Observed
It looks like the rule for accepted input is having any character (including space), followed by `@`, followed by any char again:

https://user-images.githubusercontent.com/73365754/112228005-367b3200-8c39-11eb-8ec6-667ca550fce0.mov

### Reproduced
1. Open a note
2. Open Collaborate dialog for the note
3. Enter ' @ ' into email field and submit it ~> it's just a tag (with spaces)

### Where did you see the bug
Not OS/browser dependant, but for example:

- System Make: Parallels Desktop on MBP 16"
- OS: Windows
- OS version: 10 Home 20H2 build 19042.508
- Simplenote app version: 2.9.0-beta1 Desktop

Start at the Collaborate dialog's email field and reproduce the issue with ` @ ` and the other examples in the report. Trace the validation used when submitting an invitation, then verify that malformed tag-like entries are rejected while valid email addresses still work.

Written by the indexing model from the issue text.