Screen size warning

[Xananax]

Related issues:

• mobile browser wrongly detected #532
• Warning, the app is not suitable on mobile device, is not fully readable #448

Changes

• fixes the CSS of the warning so it fits on any screen
• changes the message to remove the implication of "mobile" and focus on screen size only
• changes the detection logic to not care about aspect ratio, and only trigger under two conditions: either browser window width is under 700px, or height is under 400px, or the orientation is portrait (width < height).

Note: 700px and 400px are arbitrary values. In my personal tests, they seems to be the lowest value after which texts become uncomfortable.

If we want to target mobile users more aggressively with that notice, we can use the media queries (pointer:none), (pointer:coarse), (hover:none), (hover:on-demand) (based on this SO answer

Additionally, this PR adds modifications to the run script so we can download the custom export templates locally easily