[Echo] Try and fetch new expired slots on sending; Convert all images to JPEGs

[dracos]

Hopefully resolving two reasons we get stuck/problem reports:

1. Some backends have a maximum filesize for images. This has only ever been hit by PNGs people occasionally upload. Rather than limit this to those backends or PNGs, easier to convert all uploaded images to JPEGs.
2. If a report to Echo fails due to expired slots for some reason (we do check at report creation but if there's another issue sending (e.g. issue 1 above) this could happen), try and fetch new ones immediately for next retry rather than await manual interaction.

[codecov]

Codecov Report

Attention: 18 lines in your changes are missing coverage. Please review.

Comparison is base (0eac24d) 85.34% compared to head (a92d737) 52.90%.

Files	Patch %	Lines
perllib/FixMyStreet/Roles/CobrandEcho.pm	55.55%	9 Missing and 7 partials ⚠️
perllib/FixMyStreet/Roles/CobrandSLWP.pm	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4778       +/-   ##
===========================================
- Coverage   85.34%   52.90%   -32.44%     
===========================================
  Files         341      321       -20     
  Lines       24647    23608     -1039     
  Branches     4673     4451      -222     
===========================================
- Hits        21035    12490     -8545     
- Misses       2191     9861     +7670     
+ Partials     1421     1257      -164     

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[davea]

One typo, otherwise looks good! Is there a risk that if Echo somehow always returned Selected reservations expired for every attempt a report would be tried over and over again with little throttling? Guess it's unlikely, however.