[SeoBundle] Added event to populate robots.txt

[waaghals]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Fixed tickets	n/a

I've used events to generate a robots.txt. This makes for an easy extension point for dynamically modifing the robots.txt.
I'm having doubts about the working of Kunstmaan\SeoBundle\EventListener\FileRobotsTxtListener.
It no longer uses the request's basepath. But I do not understand how it could have worked previously, as the controller would never be called when the file exists in te public directory.

[ProfessorKuma]

Hi @, your PR needs some changes

• This PR seems to need a milestone of a minor release.

[ProfessorKuma]

Hi @, your PR passed all our requirements.

Thank you for contributing!

[acrobat]

Is there a specific reason every action/check for robots file/content is moved to an event listener? I would think it would enough to keep the current behaviour and just dispatch an event at the end that would allow you to hook into the robots content and modify it or event fully replace it.

Is there a specific need for this setup?

[waaghals]

My reasoning was to allow for easier changing of the logic.
For example, lets say I would like the content from the file robots.txt to be appended to the existing content from the entity
instead of acting as a fallback.

By making smaller listeners this could easily be done by only replacing FileRobotsTxtListener. Otherwise the new implementation would also need to implement the existing logic of the Entity fetching etc.