Fix XML escaping issue

[nicksellen]

Closes #2203

What does this PR do?

Escapes XML content when creating GPX markers.

Links to related issues

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined chat.foodsaving.world/channel/karrot-dev
• added an entry to CHANGELOG.md (description, pull request link, username(s))

[codecov]

Codecov Report

Merging #2204 into master will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2204      +/-   ##
==========================================
+ Coverage   57.42%   57.49%   +0.07%     
==========================================
  Files         279      280       +1     
  Lines        5621     5635      +14     
  Branches      841      841              
==========================================
+ Hits         3228     3240      +12     
- Misses       2392     2394       +2     
  Partials        1        1              

Impacted Files	Coverage Δ
src/maps/export.js	85.71% <100.00%> (ø)

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