BREAKING CHANGE: Setup refactor and code cleanup

This MR makes several major tweaks to the codebase. Primarily it adjusts
the setup steps for the application so that rather than providing just
the project ID in the `.gitlab.nvim` file, users can also provide a
vareity of other settings, such as auth_token, base_branch, and so
forth. This is to make the project more extensible in the future.

This MR also fixes a variety of issues with error handling in the code,
primarily in the request/response model between the Lua jobs and the
Golang server.

BREAKING CHANGE: Modifies `.gitlab.nvim` and setup steps

lua/gitlab/discussions.lua

@@ -18,7 +18,10 @@ end
 
 M.send_reply       = function(text)
   local escapedText = string.gsub(text, "\n", "\\n")
-  local json = string.format('{"discussion_id": "%s", "reply": "%s"}', state.ACTIVE_DISCUSSION, escapedText)
+
+  local jsonTable = { discussion_id = state.ACTIVE_DISCUSSION, reply = escapedText }
+  local json = vim.json.encode(jsonTable)
+
   job.run_job("reply", "POST", json, function(data)
     local note_node = M.build_note(data.note)
     note_node:expand()