Added ability to save `about:` pages.

[amohamed11]

Hi, I've been using Amfora for short-while and wanted to contribute to this awesome project.
Proposed Changes:

• Extracted the about page check that is in hasContent to its own separate function as that was the only check prohibiting about page saves. Since a page's content has no implication on it being an about page, decoupling these checks seems like a good idea. But do let me know if this change does not make sense for the overall project, and I can look into other fixes.
• Added a change to downloadNameFromURL so that when it's generating a filename for a saved about page, it uses the entire page url i.e. about:subscriptions as otherwise the filename was simply left blank.

Fixes #210

P.S. I did not locate a contribution guide, so let me know if I went about something the wrong way. If there is such a guide, please do point me towards it.

[makew0rld]

Thanks for the fix! Note I made a small change in 56b7e4a, because otherwise URLs like about:subscriptions?2 would be saved as subscriptions?2.gmi, and ? is a banned filename character on Windows.

Otherwise looks good, thanks for your work!