Migrate to Transifex Go client

.tx/config

 [main]
-host = https://www.transifex.com
+host     = https://www.transifex.com
 lang_map = zh-Hans: zh-hans
 
-[briar.briar-quick-start-guide]
-source_file = content/_index.md
-file_filter = content/_index.<lang>.md
-type = GITHUBMARKDOWN
-mode = developer
-source_lang = en
+[o:otf:p:briar:r:briar-quick-start-guide]
+file_filter  = content/_index.<lang>.md
+source_file  = content/_index.md
+source_lang  = en
+type         = GITHUBMARKDOWN
 minimum_perc = 100
+