Added absolute index flag to staticdir

[nils-ost]

What kind of change does this PR introduce?

• bug fix
• feature
• docs update
• tests/coverage improvement
• refactoring
• other

What is the related issue number (starting with #)

#1952

What is the current behavior?

Please refer to #1952

What is the new behavior (if this is a feature change)?

As proposed in my feature request I added the argument abs_index to the staticdir function. The default value is False, which does not change the behavior of staticdir as is. But if abs_index is set to True the given index -file is no longer searched relative to the Request-URI. Instead it is searched directly under the path given in dir argument.

Checklist:

• I think the code is well written
• I wrote good commit messages
• I have squashed related commits together
• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences