[web] Make all <a> and <button> cursor:pointer

Summary:
This came up in D3364 when we manually added cursor:pointer to a.sendButton.

Followed @palys-swm suggestion (https://phabricator.ashoat.com/D3364#90810) and applied this style "globally."

Note: I am not planning on going through the codebase right now and cutting every extraneous cursor:pointer applied to an <a> or <button>. I think doing so would be fine, but I don't want to check every single instance right now to ensure that there are no regressions. I think it's fine to leave them in even if they may be redundant.

Test Plan:

1. Patch this diff.
2. Revert change in D3364 (comment out line manually)
3. Make sure that we see a pointer when we hover over sendbutton
4. Revert this diff
5. Make sure that we don't see a pointer when we hover over sendbutton

Reviewers: palys-swm, def-au1t, varun, benschac

Reviewed By: palys-swm, benschac

Subscribers: ashoat, Adrian, karol-bisztyga, palys-swm

Differential Revision: https://phabricator.ashoat.com/D3371