adapt license files + format about:license (fix #11655)

[moving-bits]

Description

• adapts license files as described in Missing date and name of the copyright owner in application license  #11655
• applies Markdown formatting to about:license page for better readability
• and, as I came along while working on those issues, I removed references to our old IRC channel as well

[SammysHP]

I don't think it will work this way. The license file can stay as it was, maybe just the appendix removed. But every source file requires the header.

I suggest to add the template from the appendix to the readme and at the top of the license page in the app.

[moving-bits]

Thanks @SammysHP for reviewing. I adapted the PR accordingly:

• removed appendix from license files
• added info about c:geo team etc. at the beginning of about: contributors page
• added info about c:geo team etc. in our README
• and BTW removed the link to the old Freenode chat

[bekuno]

With e05ffa9 the file https://github.com/cgeo/cgeo/blob/e05ffa94276a7938e85bb5b5b5e2b1229f186770/NOTICE was created to cover the license information in c:geo. Now this will be added to the README.
Can we remove the NOTICE (I did not find where it is used in c:geo or indirect used by GitHub)? If not, then the year should renewed.

[moving-bits]

Ok, I've updated the PR to delete the NOTICE file

[bekuno]

The NOTICE file is only necessary for ASF projects, see e.g. github/choosealicense.com#701 . So it is fine to remove it.

[moving-bits]

retest this please