Possible attachment previewer improvements

[ThiefMaster]

based on feedback from various users. to be discussed first

• User setting to disable it completely. The drawback is that this would not only disable it e.g. for PDF files (which is the previewer most of the negative feedback was about, but that might also be because PDF is one of the most common file types in indico) but also types where it's very useful such as Markdown or even images.
• Full-width PDF display, e.g. suggested in snow RQF0483784. The reasoning for not going full-width there was to keep the style more in line with the other previewers (having a full-width PDF preview will always "hide" the page's contents while the other previewers simply gray it out).
• Can't close PDF preview with ESC. Tested in Firefox. Might be unfixable though in case the PDF viewer catches ESC keypress events.

[pferreir]

I am mostly against a user setting, primarily because there are super simple workarounds for purists (i.e. just use the middle mouse button or equivalent) but also because I have the feeling we will regret it sooner or later. People will get used to having that option and will get angry if in the future we need to remove it.