Based on the idea of https://github.com/umbraco/Umbraco-CMS/blob/release-7.3.2/src/Umbraco.Web/IUmbracoContextAccessor.cs & https://github.com/umbraco/Umbraco-CMS/blob/release-7.3.2/src/Umbraco.Web/SingletonUmbracoContextAccessor.cs

I've implemented an "UmbracoApplicationContextAccessor which allows readonly access to the UmbracoContext & ApplicationContext. In most cases this will use the SingletonUmbracoApplicationContextAccessor but in the case of tests there is a publicly available static register method to setup an alternative which used a mocked version of this accessor.

Not happy about the accessor being passed through so many properties but it's the best way I can think of to ensure we're using exactly the same contexts from method to method.

Conflicts:
	src/Our.Umbraco.Ditto/Extensions/PublishedContentExtensions.cs

Removed the static class, UmbracoDictionaryHelper.

Tentatively added in UmbracoHelper instance as in Umbraco v7.3.2 this is the best way to get a dictionary value.

Also worked in some teardowns for Resolution.Reset() so tests work between this and the UmbracoPicker tests.

Finally, there's a slight change to the dictionary tests as UmbracoHelper.GetDictionaryValue doesn't do an ordinal key check for keys. Not sure how we can fix this within Ditto, it might be a feature Umbraco adds in later on.

Conflicts:
	src/Our.Umbraco.Ditto/Extensions/PublishedContentExtensions.cs
	tests/Our.Umbraco.Ditto.Tests/Our.Umbraco.Ditto.Tests.csproj