separate settings and manual scan in admin ui

[stklcode]

First step for a UI rework. Addresses #22 and is based on the mockups by @00travelgirl00.
This PR does not introduce any new features and does not change our API, s.t. not all features from the mockups are available yet (theme grouping, last scan timestamp, etc.)

Manual theme file scan is not placed in a separate tab.

Settings table uses default WP settings styles and the scan results are presented in a WP-like table structure.
(could be migrated to settings API which would ease real modularization, but not now)

The first column is yet intentionally left empty. In a followup step I'd like to see grouping the files by theme (or maybe file system hierarchy), so toggle controls should be placed there.

Preview (animated)

[Zodiac1978]

This looks great!

Just one idea: Maybe we could add a brighter version of the highlight color as a background-color for the row to make it easier to see what line is processed and how is the result (like on the plugins page).

For example:

.av-status-ok {
  background-color: #edfaef !important;
}

[stklcode]

Good idea. We should darken the font color to the 70% values though, to keep a good contrast ratio and override the default opacity of the code-box, s.t. it's not a redish gray.

Like so:

[codecov-commenter]

Codecov Report

Merging #107 (3124506) into develop (b6edc75) will decrease coverage by 1.84%.
The diff coverage is 0.00%.

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@@              Coverage Diff              @@
##             develop     #107      +/-   ##
=============================================
- Coverage      35.93%   34.10%   -1.84%     
- Complexity       151      155       +4     
=============================================
  Files              5        5              
  Lines            782      824      +42     
=============================================
  Hits             281      281              
- Misses           501      543      +42     

Impacted Files	Coverage Δ
inc/class-antivirus.php	16.06% <0.00%> (-1.39%)	⬇️

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[stklcode]

Rebased on top of the latest develop state.

Tested on a real-world 5.9.3 site without any issues. Still looks like the previously attached screenshot.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
2 Code Smells

No Coverage information
0.0% Duplication

[Zodiac1978]

I do not check every single line, so this needs more eyes to check after merging.
LGTM so far.