Tasawer/242/site notifications

[tasawernawaz]

Pre-Flight checklist

• Screenshots and design review for any changes that affect layout or styling
  • Desktop screenshots
  • Mobile width screenshots
  • Tag @Ferdi or @pdpinch for review
• Migrations
  • Migration is backwards-compatible with current production code
• Testing
  • Code is tested
  • Changes have been manually tested

What are the relevant tickets?

Fixes #242

What's this PR do?

Add feature for site notifications.

How should this be manually tested?

• Add new notification through cms/snippets/cms/sitenotification/
• Visit any page, you should see notification.
• Notification should disappear for the rest of the session.

Any background context you want to provide?

RFC: #306

Screenshots (if appropriate)

Wagtail:

Rendered:

Out of scope:

• date range between which the content should be displayed
• ensure that only one site wide notification can be displayed at once (@pdpinch I'm guessing we want this to avoid overwhelming the user?)

[codecov-io]

Codecov Report

Merging #309 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
+ Coverage   94.03%   94.05%   +0.01%     
==========================================
  Files         170      171       +1     
  Lines        5771     5787      +16     
  Branches      342      343       +1     
==========================================
+ Hits         5427     5443      +16     
  Misses        280      280              
  Partials       64       64

Impacted Files	Coverage Δ
mitxpro/templatetags/render_notification.py	100% <100%> (ø)
cms/models.py	93.02% <100%> (+0.4%)	⬆️
cms/factories.py	100% <100%> (ø)	⬆️

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[rhysyngsun]

Generally looking good, just a few small things.

[tasawernawaz]

@rhysyngsun Suggestions applied.