Theme import & export

I took a basic shot at implementing #119 (closed).

WIP because I'm not sure about the styles for the buttons, they look rather weird (screenshot), but I wasn't sure how I should change the styles to conform to the rest of the site. Any feedback would be nice.

TODO:

• Button styles
• Feedback when an invalid file is selected (either not JSON or not a valid theme object)

changed the description

changed the description

added 2 commits

• 8f791938 - Unify button styles and use min-width
• 2a87e291 - Add validation of the imported theme and the corresponding warning message

Compare with previous version

marked the checklist item Feedback when an invalid file is selected (either not JSON or not a valid theme object) as completed

marked the checklist item Button styles as completed

unmarked as a Work In Progress

changed the description

I went ahead and unified the button styles into the global styles, it seemed to be duplicated across a lot of components needlessly. While doing that I also changed the static width that was set on them to a min-width, this keeps the look of them where the text is short enough just like they were before the change, but it allows them to get wider if the text is longer. This is probably helpful for localization, e.g. German tends to be long.

I also added a version key to the exported theme files, this should prevent headaches if there is a more advanced version of this in the future.

merged

mentioned in commit 9982376f

@elomatreb @kaniini @lambadalambda the whole "replace width with min-width" doesn't really do any good, tbh.