Move loading spinner to modal body

[rmainwork]

This PR moves the loading spinner from the modal footer...

.... and into the modal body

this fixes a UI bug where saving meetings causes the modal footer (and buttons) to stretch vertically. It also hides the modal content when saving, which is how other modals behave, so it also provides some consistency.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have run eslint on the code
• [ ] I have added JSDoc for all of my code (where applicable)
• [ ] I have added tests to cover my changes.

Priority:

• [x] Normal
• [ ] High

Related Issues:

Fixes #595

[codecov[bot]]

Codecov Report

Merging #611 (7f0ea34) into develop (fe2b990) will decrease coverage by 0.04%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #611      +/-   ##
===========================================
- Coverage    95.28%   95.23%   -0.05%     
===========================================
  Files          191      191              
  Lines         4472     4470       -2     
  Branches       540      539       -1     
===========================================
- Hits          4261     4257       -4     
- Misses         100      101       +1     
- Partials       111      112       +1     

Impacted Files	Coverage Δ
...c/client/components/pages/Courses/MeetingModal.tsx	92.35% <100.00%> (ø)
src/server/location/location.service.ts	93.10% <0.00%> (-3.45%)	:arrow_down:
...rc/client/components/pages/Courses/tableFields.tsx	92.66% <0.00%> (-0.14%)	:arrow_down:

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