Fix sticky alerts being covered by top navbar when scrolling content.

[sayravai]

Fixes #1489

Description

Add an offset of 60ox to accommodate BS5 sticky toolbar.

Why?

Course sticky notes were covered by the top navigation bar when scrolling content down.

How?

A small change in main.cs and the jQuery script on course_base.html.

Testing

Note: if there are multiple sticky-alert, they will initially all show, but when scrolling down content, the bottom one will eventually cover others. Fixing this requires bigger changes to the code and possibly some added javascript, so it is not fixed in this PR. Alternatively, the alerts could be placed under the site-messages div instead of on top of the main content.

What type of test did you run?

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

[ADD A DESCRIPTION ABOUT WHAT YOU TESTED MANUALLY]

Did you test the changes in

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