system/gnu-unifont: Updated for version 15.0.01, add TTF back to the default formats list

This merge request contains two changes:

1. Updated for version 15.0.01

Checks performed:

1. Download locations:

a. https://unifoundry.com/pub/unifont/unifont-15.0.01/unifont-15.0.01.tar.gz
b. https://ftpmirror.gnu.org/unifont/unifont-15.0.01/unifont-15.0.01.tar.gz
c. https://ftp.gnu.org/gnu/unifont/unifont-15.0.01/unifont-15.0.01.tar.gz
d. ftp://ftp.gnu.org/gnu/unifont/unifont-15.0.01/unifont-15.0.01.tar.gz

2. Files' signatures match publisher's public key:

http://unifoundry.com/1A09227B1F435A33_public.asc

3. Files from all 4 sources are identical.

Size:            27490811 bytes
MD5 checksum:    d6366a8fd03e815c2f18d36ff1a5cca7
SHA256 checksum: 7d11a924bf3c63ea7fdf2da2b96d6d4986435bedfd1e6816c8ac2e6db47634d5

4. VirusTotal checks:

a. https://www.virustotal.com/gui/url/74d7793ed3266223774bbe935be705c3adaafbc80efd23ac6e8d8726c482cb1f/summary
b. https://www.virustotal.com/gui/url/d0db2e38b9264a73fd281d69cffce982603f82d1e6c44ce6606113496b1af9c9/summary
c. https://www.virustotal.com/gui/url/989b5c59400104e8f7e6a5faea8681440e89b75dfe3fd3f53d4113730bd4d7eb/summary
d. For some reason VirusTotal didn't want to scan FTP URL.

Checks a-c passed.

5. Passed scan using the latest version of ClamAV.

2. Added TTF back to the default formats list

TTF format was removed from the default formats list as the fonts in this format are not going to be supplied anymore by the upstream starting from Unifont 16.0.01.

However, there still exist packages that depend on the TTF format of this font. Because of this, the removal of TTF format from the default formats list has been reverted in this commit.

3. Updated homepage in README and slack-desc to https