Change date_time validation from regex to DateTime parser

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When executing the PR #634 on production we noticed that this string 2017-02-04 21:05:15 +0000 was not recognized as a valid string.

Error Example

osm_id: 21302157
EXIF date_time format '2017-02-04 21:05:15 +0000' is not supported: no implicit conversion of Time into String

Even though this was fixed by using to_s on date_time, we decided to refactor the current validation to be on the safe side for the future. Luckily the DateTime parser would parse most of the date_time formats we need for now.

Allowed date_time formats:

• "2017/07/10 5:21:29 AM"
• "2017/07/10 5:21:29 pm"
• "2017/07/10 5:21:29 p.m."
• "2017-07-10 5:21:29 a.m."
• "2017/07/10 5:21:29 P.M."
• "2017-07-10 5:21:29 +2000"
• "2017-07-10 05:21:29 -2000"

This is a follow-up PR of PR #634 and part of Github issue #633.

[1000miles]

Thaaaaank you for review, @justahero ! 🍰

[1000miles]

Deployed to production today: https://github.com/sozialhelden/wheelmap/releases/tag/production-20170710114323