Use current time in location consistently

[grubbins]

Should resolve #108

[grubbins]

I have deployed the plugin with these changes, and it has fixed all the known incorrect results. So far everything is now working :)

Beyond this change, I think it would make sense to calculate time.Now().In(location) only once for each remind request and then propagate the result through all the rest of the calculations (maybe attach it to the request object?). Two reasons:

1. Efficiency: we retrieve the location and invoke time.Now().In(location) multiple times for each reminder request
2. Correctness: the result of time.Now().In(location) may change during the handling of the request, in which case we will get a broken result. We should perform all calculations with reference to the time of making the request.

However, that's a fair bit more work so maybe we could merge this PR and address the above changes separately?

[scottleedavis]

This is great, thank you @grubbins. 👍

I would be open to a separate PR to address 2 reasons you describe. Attaching the time.Now().In(location) to the request object makes sense. Aside from that, as long as the propagation of the new field doesn't cause much additional clutter, I am all for it.

[scottleedavis]

@grubbins released v0.3.1 https://github.com/scottleedavis/mattermost-plugin-remind/releases/tag/v0.3.1