Escape leading spaces and tabs in markdown processing.

When escaping plain text as markdown, we need to handle leading spaces (four or
more) and leading tabs, as these blocks are considered to be code samples. This
meant that some basic syntax highlighting would be applied (often erroneously),
and that URLs were not properly linked.

After experimenting for quite some time with escaping the first tab or space, I
gave up on that approach. Markdown renderers do not handle escaped spaces well.
The solution that I settled on was to replace the first space with a
non-breaking space entity (for leading spaces), and prepend the line with an
nbsp (for leading tabs). This renders correctly while still allowing us to
escape and unescape.

Testing done:
- Reproduced the bug (using `--description="$(git log -n 1 in `rbt post`) and
  saw that it indented the commit message without treating it as a code block.
- Used the API with `?force-text-type=plain` to verify unescaping of the nbsp
  entities.
- Ran unit tests.

Fixes bug 3503.

Reviewed at https://reviews.reviewboard.org/r/6200/