feat: image compression/resize

[schorschii]

Contribution Summary

A long wished feature from users is an automatic image compression, similar like Apple Mail and other apps do.

closes #1758, #5763

In the past, there were already multiple cancelled attempts to implement it (#2335, #3845). I'd like to start a new one since I think it would be very useful for many users.

Description & Screenshots

Sending images from smartphones is a very common task and attachment size quickly reaches provider limits with the image file size that current smartphone cameras produce.

This change adds an option "Compress Images" into the compose view. After the user has enabled the feature, it will be automatically activated in the next email too.

The JPEG quality and resolution can be configured with new options in the app settings.

If this gets merged, I'd recommend to display the menu items "Enable Encryption" and "Enable PGP Sign-Only" as checkbox too for better UX. I can make a PR for this if you like.

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• I have read, and I affirm that my contribution adheres to Mozilla’s Community Participation Guidelines
• This contribution is in Kotlin where possible
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• This contribution includes tests for any new functionality and maintains tests for any updated functionality.
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• This PR has a descriptive title and body that accurately outlines all changes made and contains a reference to any issues that it fixes (e.g. Closes #XXX or Fixes #XXX).

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