this commit fixes a regression in ecb09a0d.

- various cleanup and split of code to make it more testable
- start writing some tests

Don' request netscape.security.PrivilegeManager.enablePrivilege("UniversalFileRead") if netscape.security.PrivilegeManager is undefined.
File upload tests will fail, and it seems the only way is to swith to WebDriver.

But not source_reference / destination_reference which are not relation

This way the checked uid are properly updated before the action is rendered.
TODO: factorize code in Base_do* script

If a column is explicitly resolved to a table, it cannot be resolved to
another later. This causes problems when writing a related key targeting
another table with the same column name.
This code duplicates work done in the (only) caller (in
SearchKey/RelatedKey.py), as it already registers the latest relation
table with the column to apply on it.

discussion thread view using passed from request new unindex discussion
post url.

Allow that not index (yet ) discussion posts to be show under a discussion thread view using passed from request new unindex discussion post url.