Add check for filename template values

[jerinpetergeorge]

Type of PR - Enhancement

Description

This PR will check whether the values of DBBACKUP_FILENAME_TEMPLATE and DBBACKUP_MEDIA_FILENAME_TEMPLATE contains any "slashes" (/) in it (Ref: #479 ).

Why should this be added

If the filename contains slashes, the storage will create a directory out of it and the --clean flag won't work as expected.

[Archmonger]

Blocked on #483

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (765a75d) 91.21% compared to head (dd5eaf6) 91.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #484      +/-   ##
==========================================
+ Coverage   91.21%   91.33%   +0.12%     
==========================================
  Files          19       19              
  Lines         854      866      +12     
  Branches      150      152       +2     
==========================================
+ Hits          779      791      +12     
  Misses         40       40              
  Partials       35       35              

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[Archmonger]

@jerinpetergeorge Can you update the hint to also suggest changing location?

Maybe adding another sentence like Did you mean to change DBBACKUP_STORAGE_OPTIONS['location']?

I'm not sure if there's an equivalent for media storage?

This PR can be merged after that.