Condenser] Fix issue with page not loading when json_metadata tags field contains an Array of Arrays

Condenser] Fix issue with page not loading when json_metadata tags field contains an Array of Arrays
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Issue
https://github.com/steemit/condenser/issues/2254
When a user submits a comment that has an Array of Arrays for the tags field, Condenser breaks due to expecting a single array.

Solution
When parsing the tags, concat the arrays together to create a single Array from the multiples if they exist.

Summary
Due to the fact that users are able to submit arbitrary data in the json_metadata field that Condenser parses, users are able to completely break their own account by submitting improperly formatted tags in the json_metadata. One user in particular @makafuigdzivenu encountered this issue and is unable to load any of his pages via Condenser. This pull request contains a quick solution to the problem by just concatenating multiple arrays in the tags field to just a single array.

Example Of Improperly Formatted JSON_METADATA
(tags should be an Array of strings)
json_metadata: {
tags: [['badtag']],
app: 'testpost',
community: 'testpost',
format: 'markdown+html'
}
Shoutout to @ makafuigdzivenu for reporting the original issue on steemit/condenser

Screenshots
Improperly formatted tags breaking Condenser
screen shot 2018-01-17 at 1 15 06 pm

Improperly formatted tags concatenated into a single Array works fine
screen shot 2018-01-17 at 1 42 05 pm

Links
Proof of Work Commit #9053d7f
SteemLiberator Condenser With This PR Live
Condenser Github Issue #2254
Upstream Condenser Pull Request