Support for config-aware relative paths

[chriddyp]

Fixes https://github.com/plotly/dash-core/issues/78

[chriddyp]

Getting an odd testing error locally:

PYTHONPATH=~/dash/tests/assets pytest tests/unit/test_configs.py
Test session starts (platform: darwin, Python 3.6.2, pytest 5.2.2, pytest-sugar 0.9.2)
cachedir: .pytest_cache
rootdir: /Users/chriddyp/Repos/dash-stuff/dash, inifile: pytest.ini
plugins: mock-1.11.2, sugar-0.9.2, dash-1.7.0
collecting ... 
――――――――――――――――――――――――――――――――――――――――――――――――――――――― tests/unit/test_configs.py ――――――――――――――――――――――――――――――――――――――――――――――――――――――――
ImportError while importing test module '/Users/chriddyp/Repos/dash-stuff/dash/tests/unit/test_configs.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
../dash-design-kit/vv/lib/python3.6/site-packages/_pytest/python.py:501: in _importtestmodule
    mod = self.fspath.pyimport(ensuresyspath=importmode)
../dash-design-kit/vv/lib/python3.6/site-packages/py/_path/local.py:701: in pyimport
    __import__(modname)
../dash-design-kit/vv/lib/python3.6/site-packages/_pytest/assertion/rewrite.py:142: in exec_module
    exec(co, module.__dict__)
tests/unit/test_configs.py:15: in <module>
    from dash._utils import (
E   ImportError: cannot import name 'get_relative_path'

[chriddyp]

These relative URLs are ambiguous in this scenario as we don't know what page we're on.

For example, if we were on page /my-dash-app/this-page and then provided a relative-page-1, the resolved URL should be /my-dash-app/this-page/relative-page-1 but we can't provide that to the front end because we are unaware of this-page.

So, I thought we'd just keep it a simple prefix without any additional logic.

Relative URLs aren't documented in Dash nor used very much anyway.

[chriddyp]

Naming options:

• get_relative_path
• get_relative_url
• get_rel_path
• get_rel_url
• relative_path
• relative_url
• rel_path
• rel_url
• relpath
• relurl

[chriddyp]

• It is technically a path and not a full URL although folks might think that "path" is corresponding to the file system path.
• We use get_ in other places like get_asset_url. Do we keep it for consistency? I'd prefer to move get_asset_url usage over to this function, so folks just need to learn e.g. app.relpath('/assets/logo.png') and app.relpath('/page-2')

[chriddyp]

ugh I rebased off of master instead of dev