Add check version functionality

[bnkai]

Adds initial support for new version checking. Mentioned also in #288
As it is now it uses the github api to get the shorthash commits from the repo tags.
It matches the current version ( v0.0.0-alpha or 'v0.0.0-alpha_dev' ) to the respective repo tags and gets the related commit hash from there.If it's different than the local that means that a new release build is available.
It first prints a message in the console when the app is started and you can see it also when you open the about tab from the settings.An extra button is also available for manual checking.

for dev testing the build , since version is added through TRAVIS you need to add it to the ldflags in the Makefile
-X 'github.com/stashapp/stash/pkg/api.version=v0.0.0-alpha_dev'
or
-X 'github.com/stashapp/stash/pkg/api.version=v0.0.0-alpha'

[Leopere]

Eventually, I really would like to update automagically by default. Potentially allow people to disable the update mechanism via some command line launch argument or something not in the UI.

[WithoutPants]

Reviewed but not yet tested.

[WithoutPants]

Query and minor changes.

[WithoutPants]

Since we are getting the URL anyway, is there any reason we can't or shouldn't include a direct link to the latest download in the about page?

[bnkai]

Since we are getting the URL anyway, is there any reason we can't or shouldn't include a direct link to the latest download in the about page?

The API endpoint i am quering is this one tags.
It only has the versions and the URL of the zipfile , tarball source code no releases.
There is also the releases one that can be queried , it is more complex to parse though.
I'll have a look at it , switching to WIP till i settle this.

[ueaslsef]

The download path should be rather straight forward: https://github.com/stashapp/stash/releases/download/$version/stash-$platform. Maybe we could cheat and skip the releases-api parsing all together and just use this until we found a better way? It could even be a static string inserted by the ci on build time.

[bnkai]

@ueaslsef since we are doing it we might as well go all the way ..
The releases API has extra stuff also like times downloaded or date released. Do we want any of that ?

[bnkai]

squashed and rebased , also fixed a merge conflict due to latest commits.
this is the latest version with the proper release added as download link.

[WithoutPants]

Post-test review.

I'm thinking that the latest version section should only be shown if the version tag is set. As it is, when no version is set (ie local build), it shows an empty Latest Version Build Hash and a button that does basically nothing.

[WithoutPants]

Looks good.