Add a new Downloads tab to Family pages

[AntonPetrov]

Several users pointed out that it's difficult to find where to download various family specific files, so I suggest creating a new Downloads tab on the left hand side of Rfam family pages. It would aggregate all the downloadable items that are spread across all the other tabs, such as:

• Covariance model
• All FASTA sequences
• Sequences as a table
• Seed alignment in various formats
• Seed tree
• Species tree
• Secondary structure diagrams as images

There may be other downloadable items that I missed so please click around the family pages to double check.

We can keep the existing download links as they are or drop them in favour of the new tab (this can be decided on a case by case basis).