Encapsulate all 123done.org locators into tests/base.py

While making changes to the bidpom tests due to a change to 123done.org, I noticed that locators for the app were hardcoded all over the place. We should follow best practices and put them in one place.

I could have created a page object for the 123done.org home page, but I didn't want to mix that (which is only needed for the tests) with the actual bidpom page objects, which are used by other suites. I could have put the page object somewhere else, e.g., /tests/pages, but it seemed simpler to just put the locators into tests/base.py.

Feedback is welcomed.

Pinging @davehunt on this as he asked me to do so in irc earlier today when this came up.