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Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Closes #13905

Rails adds an empty error to the Errors object even if you just call
`#[]` on it:

```
[1] pry(main)> u = User.last
[2] pry(main)> u.errors.keys
=> []
[3] pry(main)> u.errors[:username].present?
=> false
[4] pry(main)> u.errors.keys
=> [:username]
```

Prior, if the user enabled 2FA, then disabled it and came back some time
after the grace period expired, they would be forced to enable 2FA
immediately.

- Avoid multiple calls to `validates` for the avatar attributes.
- In a cropping process, don't check if the model inherits `User`, check if it responds to `:avatar_crop_size`.

Also:
- Get rid of legacy :strict_mode
- Get rid of custom :email validator
- Add some shared examples to spec emails validation

Currently any spam detected by Akismet by non-members via API will be logged
in a separate table in the admin page.

Closes #5612

Closes #201 - two-year-old bug, woo! 💥 🎉

This adds a ability to use multiple different authentication token
fields in other models. From now on it is necessary to add
authentication token field manually in each class that implements this
mixin.

This won't work efficiently if you happen to have a lot of projects.

These changes are based on those from commit
03f5ff75, except they use a UNION
instead of plucking IDs into memory.

These methods no longer include public groups/projects (that don't
belong to the actual user) as this is handled by the various finder
classes now. This also removes the need for passing extra arguments.

Note that memoizing was removed _explicitly_. For whatever reason doing
so messes up the users controller to a point where it claims a certain
user does _not_ have access to certain groups/projects when it does have
access. Existing code shouldn't be affected as these methods are only
called in ways that they'd run queries anyway (e.g. a combination of
"any?" and "each" which would run 2 queries regardless of memoizing).

This new setup no longer loads any IDs into memory using "pluck",
instead using SQL UNIONs to merge the various datasets together. This
results in greatly improved query performance as well as a reduction of
memory usage.

The old setup was in particular problematic when requesting the
authorized projects _including_ public/internal projects as this would
result in roughly 65000 project IDs being loaded into memory. These IDs
would in turn be passed to other queries.

This removes the need for plucking any IDs into Ruby.

This allows retrieving of the list of authorized projects using a single
query, without having to load any IDs into Ruby. This in turn also means
we can remove the method User#authorized_projects_id.