Use BootstrapInterface instead of phpdotenv for global parameters

[iadj]

Suggested solution for the issue reported at #377

phpdotenv doesn't always work. Env variables sometimes return null on page load. Laravel has also stopped using phpdotenv variable loading for this particular reason. See issues:
vlucas/phpdotenv#109
vlucas/phpdotenv#76 (comment)

I think we should use Yii2's BootstrapInterface to load global variables. It's good to use as many 'vanilla' Yii2 features as possible to decrease dependencies on third party packages, and it will work 100% of the time rather than 90% of the time compared to phpdotenv.

See top answer for integration: http://stackoverflow.com/questions/27778477/how-to-make-custom-settings-data-available-globally-in-yii-2

[trntv]

We can't use yii bootstrap process here, because it is invoked after config file has been loaded.
We need our settings inside config file, so they should be set before execution enters it.

[iadj]

@trntv Good point. Dotenv also appears to work properly when on a webserver. It only has issues locally. Is there some sort of fix we can implement i.e. setting a timeout or refresh period until dotenv is read?