Differentiate URS pages from URS_taxid pages

[AntonPetrov]

We need to clearly differentiate between URS and URS_taxid pages using the following logic:

• If a URS has only 1 taxid, redirect to a URS_taxid page (this should work for all the URS pages that are already indexed by Google)
• if a URS has multiple taxids, show a page listing the URS_taxid pages:

Examples

• miRNA with 1 species (redirect): http://localhost:9000/rna/URS000075A3E5/
• SRP with 5 species: http://localhost:9000/rna/URS00000478B7/
• tRNA with 38 species: http://localhost:9000/rna/URS00001DA281

Fixes #357

@blakesweeney @carlosribas - please feel free to give feedback after 🎄

[AntonPetrov]

This requires more work so closing the pull request for now.