Rudimentary framework for External Link Customisation

Add working link generator methods (tested on nr) for Sequence viewing,
FASTA Download, and NCBI. In order to provide more flexible url
customisation, we provide whole Database objects to the link
generating methods.

Signed-off-by: Vivek Rai <vivekraiiitkgp@gmail.com>
Merge remote-tracking branch 'vivek/master' into next

* vivek/master:
  Rudimentary framework for External Link Customisation

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>