storage/parser: do not clean entry tags in storage module

[rdelaage]

This is not really a bug, but en enhancement to improve code maintainability. Currently the cleaning of entry tags is done the storage module. This is not the relevant module. This should be done in the parser module. This would need a little refactoring.

[rdelaage]

See #2556 (comment)

[rdelaage]

@jvoisin introduced it in the RSS module with #2567. It should be done also for JSON and Atom feeds. I think we can factorize by putting it in the miniflux.app/v2/internal/reader/parser module, at the end of the ParseFeed function.

[jvoisin]

reader/json is already taking care of it, and so does reader/atom

[rdelaage]

Oh didn't know, but we still need to factorize it, this avoid code duplication and if a new type of feed is added it ensure it follows Miniflux standards