This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
This commit copies `login_required.welcome_message` translation override to `login_required.welcome_message_invite_only` if the `login_required.welcome_message_invite_only` override is blank. This is done so as to ensure that login page customization remains same after [this commit](93eb0a0690).