This PR allows any/all configuration values normally declared in gridsync/resources/config.txt to be optionally overridden by means of setting GRIDSYNC_-prefixed environment variables. Environment variable names must follow form GRIDSYNC_SECTION_OPTION in order for the corresponding value to override the value of the appropriate section and option of config.txt -- for example, setting an environment variable of GRIDSYNC_DEBUG_LOG_MAXLEN=512 is equivalent to having the following entry in config.txt:

In addition, this PR also slightly modifies the behavior of populating the UI such that, if a "default" grid connection is declared (e.g., via GRIDSYNC_CONNECTION_DEFAULT=demo-grid) that grid will also always be selected/displayed first by default (thereby it somewhat easier to switch between connections in the event that multi-grid support has also been disabled).

Closes #465