Refactor action code strings into an enum-like object

[ericyhwang]

Action codes used in request.a are currently string literals in the source code, e.g. qf (query fetch) or s (subscribe). That makes the code harder to navigate and learn.

This PR refactors them into an enum-like object, such that they can be used like Action.QueryFetch.

As a bonus, it'll help if/when we find the time to switch to TypeScript, too!

Things to discuss and look at:

1. Location of the object. I created a new shared/ directory for it, since it's code shared between client and server.
2. Naming and casing of the object and its keys. For casing, I decided to go with the UpperCamelCase used by TS-style enums, but I'm open to arguments for other casing or names.
3. Completeness and correctness. I did a search for the strings and and looked at each non-test usage one-by-one, aiming to only update the strings that were actually action codes. One example of something I didn't change was the calls to Connection#sendBulk, where the strings aren't action codes directly, since they're later prefixed with "b":

   sharedb/lib/client/connection.js

   Lines 381 to 383 in e828fbb

   	this._sendBulk('f', collection, actions.f);
   	this._sendBulk('s', collection, actions.s);
   	this._sendBulk('u', collection, actions.u);

4. Speaking of tests, I decided to not update the action code string literals in tests, so the tests can continue to serve as a guard against someone accidentally changing the action codes.

[coveralls]

Coverage increased (+0.006%) to 97.416% when pulling bbeaeb9 on refactor-action-codes into 2532faa on master.

[ericyhwang]

• Move from shared/ to top level, since other shared things are at the top level.
• Switch to ACTIONS.queryFetch casing