Create an ID mapping service

[blakesweeney]

It should be possible to map RNAcentral URS ids to other sequence identifiers (Ensembl transcript ids, etc) and vice-versa. This is a bit tricky depending on the mapping desired. There are some cases that are commonly requested:

• URS_taxid -> Ensembl gene
• URS_taxid -> Ensembl transcript

One possible issue is that if the sequence the URS refers to is not part of an Ensembl gene, but only overlaps one. Generally people want the ensembl gene anyway, but that will need to be dealt with and clear to the users.