Base64Field is one of the oldest pieces of code in Djblets,

responsible for taking arbitrary string data and storing it as Base64

content in the database. It relied on some older ways of working with

strings and Django fields, had very little documentation, and absolutely

no test cases.
This change updates it, bringing it in line with our modern coding and

documentation standards. In the process, it also fixes a number of

issues with the field, ensuring that value types passed in are

compatible, encoding Unicode strings to byte strings, explicitly making

Base64DecodedValue a byte string instead of a native string, and

replacing legacy cruft in the field implementation.
Unit tests were added that run through all the ways that values can be

checked and converted and makes sure that we always get the end results

we expect.

Djblets unit tests pass on Python 2.7, 3.4, 3.5, and 3.6.
Review Board test suite passes.
Built the docs and checked for formatting and build errors.