Make sitewide alerts adjustable at runtime

[ashi006]

Fixes #1047

Description

What?

Previously showing sitewide alert (e.g. about service breaks) is set in the settings file, meaning that A+ needs to be restarted for it take effect.

Why?

Setting alert in settings file is unnecessarily complicated e.g. in Kubernetes environment. This could be implemented it in such way (e.g. using DB) that an A+ user with sufficient privileges can update it during runtime, without needing to restart the whole Django app.

How?

The solution created a model named "Site alerts" in admin page where admin can add one or multiple alerts along with their status (Active / Removed). Alerts with Active status will be shown on A+.

Fixes #1047

Testing

Remember to add or update unit tests for new features and changes.

• How to test your changes in A-plus
• How to test accessibility

What type of test did you run?

• [ ] Accessibility test using the WAVE extension.
• [ ] Django unit tests.
• [ ] Selenium tests.
• [ ] Other test. (Add a description below)
• [x] Manual testing.

The issue was tested by adding multiple entries in "Site alerts" model, all alerts with Active status were being displayed on A+. No alerts are displayed if there are no entries in "Site alerts" model at all or if there are no entries present with Active status.

Did you test the changes in

• [x] Chrome
• [ ] Firefox
• [ ] This pull request cannot be tested in the browser.

Think of what is affected by these changes and could become broken

Translation

• [x] Did you modify or add new strings in the user interface? (Read about how to create translation)

Programming style

• [x] Did you follow our style guides?
• [ ] Did you use Python type hinting in all functions that you added or edited? (type hints for function parameters and return values)

Have you updated the README or other relevant documentation?

• [ ] documents inside the doc directory.
• [ ] README.md.
• [ ] Aplus Manual.
• [ ] Other documentation (mention below which documentation).

Is it Done?

• [ ] Reviewer has finished the code review
• [ ] After the review, the developer has made changes accordingly
• [ ] Customer/Teacher has accepted the implementation of the feature

Clean up your git commit history before submitting the pull request!

[ihalaij1]

Nice!

I made some small modifications to the code style and translations.

[murhum1]

Changed the alert field to a JSONField so translations are supported as well.