cmd/tclipd: allow creating tokens to edit the contents of a note from a Internet node

[lucasew]

This is a opt-in feature that, to be enabled, one must pass a flag (-use-set-tokens) or a environment variable (USE_SET_TOKENS) to enable.

Basically it allows the owner of a note to create edit tokens. The flow is the following:

• User creates a note.
• User creates a token for a note.
• Server gives a full URL.
• User can now set the content of that note by passing the content as body to the full URL of the previous step.
• If funnel is enabled then the user can even use the tokens through the Internet using any machine that speaks HTTP and can connect to the Internet.

In practice:

• go run -v ./cmd/tclipd -data-location /tmp/test-paste -hostname test-paste -tsnet-verbose -use-funnel -use-set-tokens
• Create a note from the Web UI then a token on the note page
• It will give you a full link like https://test-paste.stargazer-shark.ts.net/api/token/set/0d4f1364-548f-4fdd-ba08-d5e1c3503044
• Change the content of the note using HTTP requests
  • Ex: curl https://test-paste.stargazer-shark.ts.net/api/token/set/0d4f1364-548f-4fdd-ba08-d5e1c3503044 -d blah

I also abstracted away some details like the function to assert that the request is in the form format to a function to reuse in other routes (before that this logic was only used once).

BTW I hope I got the CSS right lol.