* Improve error feedback for duplicate deploy keys
Instead of a generic HTTP 500 error page, a flash message is rendered
with the deploy key page template so inform the user that a key with the
intended title already exists.
* API returns 422 error when key with name exists
* Add email validity checking
Add email validity checking for the following routes:
[Web interface]
1. User registration
2. User creation by admin
3. Adding an email through user settings
[API]
1. POST /admin/users
2. PATCH /admin/users/:username
3. POST /user/emails
* Add further tests
* Add signup email tests
* Add email validity check for linking existing account
* Address PR comments
* Remove unneeded DB session
* Move email check to updateUser
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
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