Add link to help page from /select_authority
Add link to help page from /select_authority

"Only requests made using WhatDoTheyKnow are shown" might not make sense
to new users of the app. This was highlighted during user testing [1].

[1] https://github.com/mysociety/alaveteli/issues/1445

Br3nda/alaveteli,4bic/alaveteli,TEDICpy/QueremoSaber,andreicristianpetcu/alaveteli_old,petterreinholdtsen/alaveteli,datauy/alaveteli,codeforcroatia/alaveteli,codeforcroatia/alaveteli,nzherald/alaveteli,andreicristianpetcu/alaveteli,Br3nda/alaveteli,Br3nda/alaveteli,TEDICpy/QueremoSaber,codeforcroatia/alaveteli,andreicristianpetcu/alaveteli,andreicristianpetcu/alaveteli,4bic/alaveteli,Br3nda/alaveteli,andreicristianpetcu/alaveteli_old,Br3nda/alaveteli,TEDICpy/QueremoSaber,andreicristianpetcu/alaveteli_old,petterreinholdtsen/alaveteli,petterreinholdtsen/alaveteli,datauy/alaveteli,nzherald/alaveteli,andreicristianpetcu/alaveteli_old,4bic/alaveteli,andreicristianpetcu/alaveteli,petterreinholdtsen/alaveteli,datauy/alaveteli,andreicristianpetcu/alaveteli_old,andreicristianpetcu/alaveteli,4bic/alaveteli,nzherald/alaveteli,4bic/alaveteli,TEDICpy/QueremoSaber,codeforcroatia/alaveteli,nzherald/alaveteli,petterreinholdtsen/alaveteli,TEDICpy/QueremoSaber,nzherald/alaveteli