Remove CSS and JS fields for new installs and ones an without existing value

[joshcanhelp]

Changes

• New installs will not see the Custom CSS and Custom JS fields, only a message and link to upcoming documentation.
• Installs without a value saved to these fields will see the same.
• Installs with a value in either field will see a deprecation message.

Testing

• This change adds unit test coverage
• This change has been tested on WP 5.1.1

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@a78b427). Click here to learn what that means.
The diff coverage is 94.11%.

@@            Coverage Diff            @@
##             master     #634   +/-   ##
=========================================
  Coverage          ?   38.88%           
  Complexity        ?     1241           
=========================================
  Files             ?       51           
  Lines             ?     3811           
  Branches          ?        0           
=========================================
  Hits              ?     1482           
  Misses            ?     2329           
  Partials          ?        0

Impacted Files	Coverage Δ	Complexity Δ
lib/WP_Auth0_Options.php	72.72% <ø> (ø)	28 <0> (?)
lib/admin/WP_Auth0_Admin_Appearance.php	14.75% <94.11%> (ø)	19 <0> (?)

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