Update valid path criteria

[kevinputera]

Closes #415

Sorry for the messy PR. It works on a few things, mainly:

1. Update ...Path's VALIDATION_REGEX to follow that of Title.
2. Refactor NewCommand and NewCommandParser by a bit.
3. Remove TO_ROOT_PATH constant. I believe this was created in the past due to InvalidPathException being a checked exception, which made calling AbsolutePath#fromString very troublesome. Since InvalidPathException is no longer a checked exception, let's remove the constant.
4. Update ...PathTest's test cases

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[codecov[bot]]

Codecov Report

Merging #419 into master will decrease coverage by 0.03%. The diff coverage is 83.33%.

@@             Coverage Diff              @@
##             master     #419      +/-   ##
============================================
- Coverage     67.14%   67.11%   -0.04%     
+ Complexity      519      518       -1     
============================================
  Files            98       98              
  Lines          2027     2025       -2     
  Branches        201      201              
============================================
- Hits           1361     1359       -2     
  Misses          580      580              
  Partials         86       86              

Impacted Files	Coverage Δ	Complexity Δ
...in/java/com/notably/commons/path/AbsolutePath.java	90.47% <ø> (-0.23%)	17.00 <0.00> (-1.00)
...in/java/com/notably/commons/path/RelativePath.java	65.95% <ø> (ø)	14.00 <0.00> (ø)
...in/java/com/notably/model/util/SampleDataUtil.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...in/java/com/notably/logic/commands/NewCommand.java	100.00% <100.00%> (ø)	2.00 <1.00> (ø)
...ava/com/notably/logic/parser/NewCommandParser.java	69.56% <100.00%> (ø)	3.00 <0.00> (ø)
...n/java/com/notably/model/block/BlockModelImpl.java	75.00% <100.00%> (ø)	9.00 <0.00> (ø)
...om/notably/storage/JsonSerializableBlockModel.java	95.12% <100.00%> (ø)	7.00 <0.00> (ø)

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