Escape tilde to avoid wrong strike-throughs in markdown

[mocobeta]

89

This just escapes all ~ in the text at the last step of the markup conversion to avoid unintentional strike-throughs in Markdown. Old-style Jira mentions (~username) also should be changed to @username not to be affected by this patch.

LUCENE-7260 is now correctly rendered without wrong strike-throughs. https://github.com/mocobeta/migration-test-3/issues/517

[mocobeta]

Escaping tildes has to be done before markdown conversion; otherwise valid strike-throughs will break.