Add content error for non-absolute width SVGs

[sjd210]

Checks all Media for SVGs containing non-absolute width (%-based width, or no width at all) via a regex match and adds it as a non-critical content error if so.

Since we don't process SVGs as content, this is the best way of dealing with this that I can think of. This means there will be no warning for SVGs not yet used as a "src", and multiple warnings for a single SVG used multiple times - but both of these are rare cases so I think it's okay.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.24%. Comparing base (9585009) to head (13966b6).
⚠️ Report is 129 commits behind head on main.

Files with missing lines	Patch %	Lines
...ava/uk/ac/cam/cl/dtg/segue/etl/ContentIndexer.java	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #753      +/-   ##
==========================================
- Coverage   38.25%   38.24%   -0.01%     
==========================================
  Files         541      541              
  Lines       23664    23670       +6     
  Branches     2822     2824       +2     
==========================================
  Hits         9053     9053              
- Misses      13735    13741       +6     
  Partials      876      876              

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