Clarify logged in registration attempt

Description

When trying to register when already logged in, make clearer that the user is already registered.

Migration and compatibility

• The merge request DOES NOT update the models. If it does, it is needed to migrate the database after merging, add the label ~"Database migrations"
• The merge request DOES NOT break the compatibility.

Related documents

(documentation and pertinent links describing the methods implemented in the merge request.)

TODO

Before being accepted, any merge request must fulfill the following requirements:

• Check PEP8 writing style as described in CONTRIBUTING.md.
• Ensure existing tests are still working properly.
• Features have been tested.
• You used your best English for every translatable string 😀.
• You marked every translatable string with gettext, blocktrans and/or trans.
• Ensure that there is no side effect in files: changes are only related to one, and only one feature.
• Ensure that no file has been removed by accident.
• Document you API, as well as the behavior, if needed, in docs.
• Ensure the commit history is “clean enough” (this means no extra merge commits, etc − try to rebase to clean it if possible).
• Ensure pipeline succeeds.

Deploy Notes

None.

Steps to Test or Reproduce

Try to sign up when logged in.

git checkout <feature_branch>  # Switch to the feature branch
git fetch --all --prune  # Download new commits
git reset --hard <remote>/<feature_branch>  # Override your local worktree with the one from remote branch

Impacted Areas in Application

Registration.