[PATCH] Inbuilt URL validation for dynamic content generated by users.

While working on the PGweb Testing Harness we noticed that although our
testing suite works on checking the entire website for broken links, we are
not able to check the real time data that is present on the main website
since the testing harness uses a development environment with some
predefined fixtures which does not aptly satisfy this particular test.

Therefore we have two patches to tackle the problem of Broken URLs at least
in the dynamic content for the pgweb repository.
1. URL validation in URL specific fields and markup fields in the forms
for adding Events, News, Organisation, Products and Professional
Services.[form_validation_with_markdown_v03.patch]

2. A secondary service which would be run as a cronjob on the prod server
to check tables that consist of specified URL fields and Markup content
fields. (I am currently using the local values for database connection in
the script; changes will be made according to the prod server if we decide
to deploy it.)[url_scan_individual_tables.patch]

Patches have been attached in the above mentioned order. Would love to hear
your thoughts on this. I think this can help us a lot in manually searching
for broken URLs on the website (at least in the dynamically generated
content). I am also working on refining the crawling service already
implemented in the Testing Harness which would be able to check for broken
URLs.