Reconsider default branch in default context

[rymsha]

Default preferred branch in different usages is different.

• To access content in live mode - master should be by default.
• To read/write non-content repositories master would be preferred (usually master is the only branch there)
• To modify by content studio - draft should be preferred. (done in Do not use ContextAccessor.INSTANCE app-contentstudio#8392)

And it does confuse developers, as they spend time understanding why when they want to read content they only get a draft version - a bug which is hard to realize earlier.
This happens because

• when hoot request is received - draft branch is selected in ContextFilter
• when context is accessed ContextAccessor.get() and context does not exist - which is a very rare case - there should be no default branch set.

Recently content's draft branch become effectively read only for most of the methods, so it is much safer to set master branch as default for http requests.

And when there is no context at all - the default branch should be not set at all - so developers can realize errors quicker.