Show application mode in footer

[kinow]

Display the application mode in the footer, when it is not equal to PRODUCTION. This is useful for troubleshooting and development.

I am planning to use a different response in the services in an mode (e.g. offline, for #75 and #77 ). This way we will be able to run the UI completely offline, with some mocked data. But also use other modes like development that could still point to development UI servers.

[kinow]

To review/test this PR, this vue.js doc page has more about modes, and it's possible to runt he cylc-ui project alone simulating modes, e.g.

$ NODE_ENV=staging npm run serve

[codecov-io]

Codecov Report

Merging #80 into master will increase coverage by 0.8%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master      #80     +/-   ##
=========================================
+ Coverage   77.41%   78.22%   +0.8%     
=========================================
  Files          13       13             
  Lines         124      124             
=========================================
+ Hits           96       97      +1     
+ Misses         28       27      -1

Impacted Files	Coverage Δ
src/store/index.js	95.65% <0%> (+4.34%)	⬆️

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[wxtim]

Happy with this.

• Common sense testing I've tested this by setting export NODE_ENV="My Name is Tim". This appears to work.
• Looking at the code

[kinow]

Thanks for testing it Tim!!!