bidi-isolate usernames in Linker::userLink

bidi-isolate usernames in Linker::userLink

Usernames can be in any script/language and include weak and neutral
characters like parentheses and dashes - these flip and make things
look very weird when displayed in the opposite directionality.

This fix adds <bdi> tags to isolate the username when we output user
links. This will affect displays like user personal tools, history
and diff view.

Also, fixing LogFormatterTestCase::removeSomeHtml() that expected
a very rigid html input (and thus failed with the new <bdi> wrapping)
to use strip_tags() instead.

Change-Id: I2db5f4b7d3a00726461eb6b699fbdf0ecd47a1cb