[RTADPA BW Tests] Embed pngs into web pages

Aims to embed png images from monitoring histograms directly into the html to a) reduce the number of files produced by the bandwidth tests and b) see if this reduces storage space.

[phrzkf@roselend embed_pngs_1_100k]$ ls Output | wc -l
19970
[phrzkf@roselend embed_pngs_1_100k]$ ls Output_embedded | wc -l
2454

i.e. factor ~8 reduction in number of files on hlt2_and_spruce_bandwidth.sh with 100k events.

The storage effect is more subtle, with differing results depending on du -s and du -b. Nominally, the base64 encoding will increase the file size by a factor ~4/3.

Comparing the ci-test of this MR and a recent nightly, an increase in storage space by a factor 295 MB / 250 MB = 1.18 is seen.

Pages built at e8ef8221: https://jconnaug-bwtests.web.cern.ch/Output_embedded/index.html

Latest ci-test: https://lhcbpr-hlt.web.cern.ch/UpgradeRateTest/BandwidthTest_lhcb-master-mr.13502_Moore_hlt2_and_spruce_bandwidth_x86_64_v3-el9-gcc13-opt+g_2025-04-26_06:16:18_+0200/