allow direct download as an attachment

[crsct]

No description provided.

[sjanssen2]

I've experienced that people struggled to find the right editor to "open" / view the downloaded *.gap files. Therefore, I prefer the browser directly to show the raw file content.
@crsct you said that there was some issue with the encoding when saving the file to disk. Is this due to the wrong mime type?

[crsct]

I checked the mime type, it is set to text/plain, if clicked as is, it opens the raw content in a new tab, which cant be downloaded by "ctrl+s" or "save as" in the context menu, this will only result in a downloaded html file and a folder with files in it (arbritary name, but content of alignments.gap).

If the point is to show them the raw file content, would it be better to have a dedicated download page that has buttons to download the file as an attachment (as it is in this PR) and a text preview akin to github?