Redirect goptlib, meek, flashproxy repositories to GitLab

(Following migration instructions.)

These repositories that I am responsible for have been moved to GitLab and need redirects. The flashproxy respository has additionally been "archived" in GitLab.

gitolite	gitlab	fate
/pluggable-transports/goptlib.git	/tpo/anti-censorship/pluggable-transports/goptlib	migrate
/pluggable-transports/meek.git	/tpo/anti-censorship/pluggable-transports/meek	migrate
/flashproxy.git	/tpo/anti-censorship/pluggable-transports	archive