Previously, we guess what syntax highlighting to apply to files in the diff

viewer given their data and file extension. Sometimes this guess would be wrong,

and we have no way of setting what type of syntax highlighting gets applied to

files. This change adds a setting for mapping file extensions to pygments 

lexers (the syntax highlighters), namely

reviewboard.admin.siteconfig.diffviewer_custom_pygments_lexers. If a mapping

is not defined for a certain file extension, we revert to the old behaviour

of guessing which lexer to use given the filename and file contents.
Adds reviewboard.admin.form_widgets.LexersMappingWidget which partly

handles the UI for modifying this mapping.

Created unit tests for the mapping in 

reviewboard.diffviewer.tests.test_raw_diff_chunk_generator.

  Ran all unit tests in reviewboard.diffviewer with success
Created the unit tests for the LexersMappingWidget in 

reviewboard.admin.tests.test_lexers_mapping_widget.

  Ran all unit tests in reviewboard.admin.tests with success.