[associations helper] Use JLoader to load the router to make association links

[andrepereiradasilva]

Summary of Changes

The component router helper should be registered in the component association helper so when loading the helper it's already auto-loaded.
So we drop jimport and use JLoader::register

Testing Instructions

In multilanguage site, component items Associations tabs work fine (business as usual) in all core components (com_content, com_contact and com_newsfeeds) that supports associations (excluding com_menus that does not need this).

Notes

This issue was discovered in the GsoC multilingual association sproject.

Note: a similiar PR will be made for com_weblinks repository.

@infograf768 @alikon @jreys please test

[alikon]

I have tested this item ✅ successfully on 8b181d7

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[jreys]

I have tested this item ✅ successfully on 8b181d7

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[infograf768]

rtc.
@wilsonge can we get that in 3.6.1 ?

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[ggppdk]

I have made same update to my extension association helper, works there too

[andrepereiradasilva]

BTW does your extension supports associations?
If so, can you post a link here: joomla-projects/gsoc16_improved-multi-lingual#29