Updates hazen-web-app to use Bootstrap5

[tomaroberts]

hazen-web-app used to implement Bootstrap3 via the Flask_Bootstrap python package. This version of Bootstrap is old and the Flask_Bootstrap package is no longer supported. Confusingly, a newer package called Bootstrap-Flask now exists, which supports Bootstrap 4 and 5. To use this, the hazen-web-app code needed refactoring.

This PR:

• implements Flask-Bootstrap
• amends app/main/base.html and a couple of other files according to the migrate instructions within the documentation
• fixes the navbar and navbar toggle button
• fixes the location of the login box
• updates requirements.txt

This should make hazen-web-app more robust going forward, and improves the functionality of the CSS which was broken. I think there are still a few corrections to make.

[tomaroberts]

Also fixed the descriptions for the Process Tasks – small bug due to changes in Bootstrap CSS syntax. Now looks like this again: