Refactor flow's config to use a minimal amount of signac's config module

[b-butler]

Description

• Remove flow.util.config.get_config_value. Removes an extant template filter.
• Make flow.util.config.require_config_value take a project.
• Rename flow.util.config.require_config_value to flow.util.config.get_config_value. This could potential trip users up if they use our template filter as now the behavior changes.
• Remove flow.environment.ComputeEnvironment.get_config_value. Use the project to get environment specific configuration values.
• Remove use of signac.common.config.read_config_file and load_config everywhere.

We no longer depend at all on signac.common.config.

Motivation and Context

signac is significantly reducing its Python config API, and this prepares for that change.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• The changes introduced by this pull request are covered by existing or newly introduced tests.
• The package documentation and framework documentation in signac-docs are up to date with these changes.
• I have updated the changelog and added any related issue and pull request numbers for future reference.

[codecov]

Codecov Report

Merging #649 (3b66da7) into master (10bbcb4) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #649      +/-   ##
==========================================
+ Coverage   79.81%   79.85%   +0.04%     
==========================================
  Files          33       33              
  Lines        3195     3187       -8     
  Branches      681      681              
==========================================
- Hits         2550     2545       -5     
+ Misses        507      505       -2     
+ Partials      138      137       -1     

Impacted Files	Coverage Δ
flow/environment.py	78.32% <ø> (-0.59%)	⬇️
flow/project.py	81.72% <ø> (-0.02%)	⬇️
flow/util/config.py	100.00% <100.00%> (+15.00%)	⬆️
flow/util/template_filters.py	68.83% <100.00%> (ø)

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[bdice]

Config changes look great. I have a few comments on the changes in conftest.py (which seem unnecessary to me) but I'm approving the core of the config-related changes.

[bdice]

@b-butler Please update the changelog and then this can be merged.

[vyasr]

Thanks! This LGTM

[shihkual]

Everything looks good. Thanks!