Allow slashes in project names

[lkysow]

Allow the project name in atlantis.yaml file to contain slashes:

projects:
- dir: terraform/myservice
  name: terraform/myservice

Also in this PR:

• Validate project names to ensure they don't contain invalid characters
• Ensure the planfile names (which are generated from the project names) don't contain invalid filename chars

Notes

• With commit from @johnlinvc in Support slash in repo name #319. I've added the validation of the config file and also changed the allowed filename chars regex
• Made change recommended by @kipkoan to only remove invalid filename chars
• Fixes Atlantis.yaml improvement - Allow project name to have slashes #253

[codecov]

Codecov Report

Merging #338 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #338      +/-   ##
==========================================
+ Coverage   70.57%   70.63%   +0.06%     
==========================================
  Files          61       61              
  Lines        3643     3651       +8     
==========================================
+ Hits         2571     2579       +8     
  Misses        893      893              
  Partials      179      179

Impacted Files	Coverage Δ
server/events/yaml/raw/project.go	100% <100%> (ø)	⬆️
server/events/runtime/runtime.go	83.33% <100%> (+5.55%)	⬆️

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