Removes django-angular lib. and its usage from login form

Description

The changes in the PR remove the usage of django-angular (djng) third-party lib. as we use patched lib. and the updated lib. requires lengthy migration and currently, we are using djng only for login form.

Ticket

JIRA link: https://tasks.opencraft.com/browse/SE-3732

[Before PR merge] Completion criteria

• Remove all traces of django-angular from Ocim: Python code, JS assets, CSS code, requirement
• Make sure the log in form works and shows errors nicely (e.g. wrong password)
• Make sure the rest of the UI isn't affected

[After PR merge] Completion criteria

• Remove [our custom django-angular branch|https://github.com/open-craft/django-angular] and any open PRs, or replace the README to say that it's outdated and that we won't use it
• Deploy on stage and production server

Testing instruction

• Run the ocim backend server (vagrant ssh followed by make run.dev)
• Open http://localhost:5000/login
• FIllup the login form with valid/invalid data for testing
• Open https://stage.manage.opencraft.com/login to compare the UI.

UI Screenshots

Earlier

After PR changes

Note for reviewers:

The 2nd screenshot in "Earlier" and "After PR changes" are different i.e, "After PR changes" doesn't show a "green tick" around the username because of two reasons:

1. It (someway) indicates the username field value is correct which is not true.
2. This indicates a client-side validation earlier rendered using djng-angular now we will have to write separate indicators for password and username.

I'm fine with adding such a (green tick) indicator for the username field if needed, open to suggestions!