Intermediate show and validate pages

[nidico]

If a user wants to perform an action which needs instance membership or email validation, but the user lacks either of those, the system shows an overlay if possible, or a 403 page if not possible (e.g. in an overlay) until now.

This pull request changes this behavior: Instead of showing a 403 page, the user is redirected to an intermediate page asking the user to join / validate their email address. The came_from information is preserved.

[xi]

Here is what I did for testing:

• Click on "create comment"
• I get redirected to login page
• I click register, fill out the form and submit
• I get redirect to ask_validate (which is strange because I just registered)
• On the ask_validate page there is an attention getter which duplicates the functionality of the page
• After I clicked on the validation link I was validated, but not logged in
• When I reloaded the ask_validate page after I validated the account it still asked me to validate my email address. It would be nice if it had deteced my validated email address and redirected me to came_from.
• On the pending_activate page there is an alert which duplicates the functionality of the page

[nidico]

I get redirect to ask_validate (which is strange because I just registered)

We should adapt the wording in ask_validate to take that case into account.

On the ask_validate page there is an attention getter which duplicates the functionality of the page

True, we should skip that if we're on that page, as we already take care of that issue.

After I clicked on the validation link I was validated, but not logged in

That's a bug (in my opinion), but shouldn't be caused by this PR. (#787)

When I reloaded the ask_validate page after I validated the account it still asked me to validate my email address. It would be nice if it had deteced my validated email address and redirected me to came_from.

True, this would be nice. I only tested going forward from the browser tab which opened from the verification email.

On the pending_activate page there is an alert which duplicates the functionality of the page

Yeah I wasn't really sure what to do about that site.