Compress images before uploading as attachment

[stefan-niedermann]

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When uploading images, they should be compressed to a sane size before doing so.

This will

1. Speed up the upload process
2. Use less data (in case you are on a mobile connection)
3. Reduce the file size on the server

Depends on #289 because this must be implemented optionally, so one is still able to upload the complete image.

[stefan-niedermann]

Will be shipped (enabled by default) with 1.15.8.