issue2551099 - disable processing of data url's in markdown. Display as plain text. Added 'data' to templating.py _disable_url_schemes array. User should be able to re-enable it by changing the array from the tracker's interfaces.py. Markdown tests failed before the change to _disable_url_schemes. Also add second separate data test for ReST and plain text processing. data url's look like they are already ignored in these proess streams.

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