Update format customizations handling

[cdubz]

@Amith211 👋 -- I'd appreciate your feedback on this branch. This moves the format handling to middleware because there seems to be no agreeable way to make this work with format modules.

Re: the 12h vs. 24h display -- Django's default formats for en-GB actually use 12h format. The USE_24_HOUR_TIME_FORMAT setting needs to be implemented per-locale when overriding the defaults. Do you think that should be necessary here? I'm not sure what the standard is in the UK but I'd be surprised if Django's defaults are not the norm.

Fixes #324.

[coveralls]

Coverage decreased (-0.02%) to 98.472% when pulling 28fe6ab on 324-en-gb-datetimes into 4b884fd on master.

[cdubz]

I'm pretty confident in this approach so I'll move it forward (want to move a 1.9.1 release but not with the en-GB unhappiness). Will address any issues in further tickets.

[Amith211]

@cdubz Apologies, I didn't get a chance to have a look the other day.

I'm not familiar enough with Python/Django to make any strong comments regarding implementation; I mainly using c#. For my understanding, would the overrides follow the same partern here, requiring a update_{locale}_date_formats() function to implement USE_24_HOUR_TIME_FORMAT for that lang?

I'll open a new issue to address the 24h format for en_GB. The short version is that for en_GB USE_24_HOUR_TIME_FORMAT needs to be implemented as this would be the expected format when shown on a digital display for most, if not all, Brits.

[cdubz]

No worries! I'm just antsy for a release (:

So yeah for customizing a particular locale you could pretty much follow exactly what was done for "en_US" in this PR. I figured 24H time made more sense for y'all and I find it very odd that Django doesn't use that as its own format default for en-GB.

[Amith211]

Yeah, odd they don't seem to follow the 'modern' standard, 12h is more common in spoken communication (this would be said as 'three in the morning/afternoon' rather than 'AM/PM'), but not for written communication, especially when formal or digital. I'll see you over at #333 so I don't get confused :)