Improve layout and copy of case search modal

[lizadaly]

A set of mostly cosmetic and copy changes to the "Add a case" modal:

• Centers the top tabs
• Removes extraneous lines and copy
• Organizes drop-downs and styles them more evenly
• Better balances the weight of the type on the modal
• Changes the advanced/basic search toggle from an anchor to an underlined button (now more obviously clickable and has the right cursor effects; should be better for a11y)
• Adds a little more breathing room around the edges of the modal content

Before

After

The above screenshots are Chrome; Firefox looks identical.

Because Safari makes styling form components hard, it looks less nice in that browser, but it also looks less nice in Safari currently:

Before

After

There will be some improvements to the actual case selection UX and functionality following this, but I wanted to separate out the copy/styling first.

[codecov-commenter]

Codecov Report

Merging #1937 (ec44c27) into develop (fda7888) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1937   +/-   ##
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  Coverage    76.88%   76.88%           
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  Files           60       60           
  Lines         6779     6779           
========================================
  Hits          5212     5212           
  Misses        1567     1567           

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