perf(sql): optimize post search query on the join with messages

[tuxpiper]

This pull request makes the following changes:

• speeds up sql query for post search , on the way that the messages table is joined
• in a specific case, I've seen the query time go from 16000+ ms to ~150 ms (100x speed-up achievement unlocked!)

Test checklist:

• Posts are still displayed on map view, data view, etc
• Perform a CSV export
• I certify that I ran my checklist

Fixes ushahidi/platform# .

Ping @ushahidi/platform

[coveralls]

Coverage decreased (-0.009%) to 20.5% when pulling 256e25f on optimize-posts-messages-join into b1b8157 on develop.

[rowasc]

makes total sense in the context of low posts to messages, lets test in steve. I'd suggest adding a note to test exports too, but other than that, not very worried.
amazing perf unlock 🙌 I

[rowasc]

this is the most useful comment block I've seen in a while, you rock

[tuxpiper]

Thanks @rowasc !

Tagging @Angamanga for consideration to add to next release