apps: Add links to donate or contribute to upstream projects

As mentioned in https://wiki.debian.org/FreedomBox/Roadmap2020. (Also see Donation field in https://wiki.debian.org/UpstreamMetadata.)

Putting together a list of links here for now:

• Deluge: https://www.patreon.com/deluge_cas
• I2P: https://geti2p.net/en/get-involved/donate
• ikiwiki: https://ikiwiki.info/tipjar/
• infinoted: https://gobby.github.io/ has PayPal donation link
• Matrix: https://matrix.org/ links to Patreon, Liberapay, and cryptocurrency addresses.
• Minetest: https://www.minetest.net/get-involved/#donate
• Mumble: https://wiki.mumble.info/wiki/Donate
• Privoxy: https://www.privoxy.org/faq/general.html#DONATE lists several ways to donate
• Samba: https://www.samba.org/samba/donations.html
• Searx: https://searx.me/static/donate.html
• Syncthing: https://syncthing.net/donations/
• Tor: https://donate.torproject.org/
• Transmission: https://transmissionbt.com/donate/
• TT-RSS: https://tt-rss.org/ links to PayPal and Patreon
• WireGuard: https://www.wireguard.com/donations/

System modules (just some key packages):

• Backups: https://www.borgbackup.org/support/fund.html
• Let's Encrypt: https://letsencrypt.org/donate/
• Pagekite: https://pagekite.net/support/faq/#donate

Adding links to packages' debian/upstream/metadata:

• Backups: debian/borgbackup!2 (merged)
• Deluge: debian/deluge!4 (merged)
• I2P: debian/i2p!1 (merged)
• ikiwiki: already has it
• Minetest: games-team/minetest!2
• Mumble: already has it
• Let's Encrypt: letsencrypt-team/certbot/certbot!4 (merged)
• Privoxy: already has it
• Searx: debian/searx!2 (merged)
• Syncthing: go-team/packages/syncthing!3 (merged)
• Transmission: debian/transmission!1 (merged)