Bug 1014219 - [Messages] Consider using real <button>s instead of <a> links for the header icons

Changes made as mentioned in the previous PR. 
Talking about test results, looks like I forgot to mention (my fault), that we have to update selector in three more places (see ```f1```, ```f2```, ```f3``` failed tests using the link above) - in python tests. Python tests will be obsoleted in favour of Marionette JS tests, but we should keep it up to date until we fully migrate.

Here are places I think we have to update too:

1. https://github.com/azasypkin/gaia/blob/6596ec05b9d00166929fe4bcda1c5c44bad4174c/tests/python/gaia-ui-tests/gaiatest/apps/messages/app.py#L13 - new selector for the composer link;

2. https://github.com/azasypkin/gaia/blob/6596ec05b9d00166929fe4bcda1c5c44bad4174c/tests/python/gaia-ui-tests/gaiatest/apps/messages/app.py#L16 - new selector for threads options button;

3. https://github.com/azasypkin/gaia/blob/6596ec05b9d00166929fe4bcda1c5c44bad4174c/tests/python/gaia-ui-tests/gaiatest/tests/endurance/test_endurance_sms_conversation.py#L17 - new selector for the composer link;