Fix ipfs gateway url to fetch dir images from other creator nodes

[dmanjunath]

What's the problem?
IPFS gateway calls for images inside directories takes the form /ipfs/Qm123/Qmabc instead of /ipfs/Qm123/<filename>.jpg

What's the solution?
Pass in the file name from the files table (original.jpg, 150x150.jpg etc...) into the saveFileForMultihash function so requests to other cnode can be in the form /ipfs/Qm123/<filename>.jpg

How is this tested?

1. Setup a fresh audius instance locally and created a user and uploaded a track
2. Set ipfs operation timeouts to 1ms so they'll timeout and it will always to go to fetch from other cnode gateways.
3. Triggered a sync
4. Verified that the number of items in both /file_storage volumes match the number of files in the files table in the db. The saveFileForMultihash function also runs ipfs add with the only has option to make sure the CID and the actual contents of the file match.
5. Clear the secondary's /file_storage volume. Do an ipfs repo gc on all ipfs nodes and make sure you can't cat the files on any node. Run steps 3&4 again
6. Add new cover art to my user profile. This should trigger a sync. Verify according to step 4 again.

[hareeshnagaraj]

let's add a QA checklist and run through some local validation before merge

[hareeshnagaraj]

prior to this fix, gateway URLS were formatted as http://cn1_creator-node_1:4000/ipfs/ipfs/QmTcm6ntbHAhz9MdzgEPFQnLzDsfw9Sb8q57gW7LTXkGnQ1 when fetching images in saveFileForMultihash

after the fix, gateway URLS are now formatted as follows
dir:

[2020-07-31T23:23:57.544Z]  INFO: audius_creator_node/746 on e3b5d3ad750c: gatewayUrlsMapped 
[ 'http://cn2_creator-node_1:4001/ipfs/QmcSqpPN5R5UheCUXMRByLHLiFvFtzFWNK4Jsww31e2evF/480x480.jpg' ] 
(requestID=GIW4YI7Fzl, requestMethod=POST, requestHostname=localhost, requestUrl=/sync)

non-dir:

[2020-07-31T23:23:57.452Z]  INFO: audius_creator_node/746 on e3b5d3ad750c: gatewayUrlsMapped 
[ 'http://cn2_creator-node_1:4001/ipfs/QmSoBan8MsFX9xLLZneoMT5243q1V3feFHD5UGz8NCzrTA' ] 
(requestID=GIW4YI7Fzl, requestMethod=POST, requestHostname=localhost, requestUrl=/sync)

[hareeshnagaraj]

LGTM