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

Changes made as mentioned in the previous PR. 
nit: line is too long here, it should be max 80 chars. To catch this you can also see the link to the test results inserted to your PR by ```try-server-hook``` - https://treeherder.mozilla.org/ui/#/jobs?repo=gaia-try&revision=39c8df13241d. Here you can see that ```Li``` (linter) failed. If you tap on failed test - you'll see details ```TEST-UNEXPECTED-FAIL | apps/sms/js/thread_list_ui.js | line 51, col 87, Line is too long.``` :)