Add fdelete fclear command

[mhdsyfq77]

Resolves #157

Added:

• ForceDeleteCommand which can be used similar to delete function but uses "fdelete" instead
• ForceClearCommand which can be used similar to clear function but uses "fclear" instead
• ForceDeleteCommandTest & ForceClearCommandTest
• Required messages for error display and help page

[mhdsyfq77]

Looks guuud; just tiny nits abt the javadocs!

And on a side note, should we write tests for ForceDeleteCommandParser? Since it's mostly a duplicate of DeleteCommandParser, perhaps we don't need to, so just asking!

yes we definitely do i totally forgot about that LOL cause if not coverage 0% xDDD

[codecov-io]

Codecov Report

Merging #178 into master will increase coverage by 0.08%. The diff coverage is 85.71%.

@@             Coverage Diff              @@
##             master     #178      +/-   ##
============================================
+ Coverage     75.40%   75.48%   +0.08%     
- Complexity      572      579       +7     
============================================
  Files            93       96       +3     
  Lines          1736     1750      +14     
  Branches        178      179       +1     
============================================
+ Hits           1309     1321      +12     
- Misses          343      345       +2     
  Partials         84       84              

Impacted Files	Coverage Δ	Complexity Δ
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...va/seedu/address/logic/commands/DeleteCommand.java	100.00% <ø> (ø)	10.00 <0.00> (ø)
...java/seedu/address/logic/parser/GradPadParser.java	69.23% <0.00%> (-5.77%)	14.00 <0.00> (ø)
...eedu/address/logic/commands/ForceClearCommand.java	100.00% <100.00%> (ø)	2.00 <2.00> (?)
...edu/address/logic/commands/ForceDeleteCommand.java	100.00% <100.00%> (ø)	2.00 <2.00> (?)
...address/logic/parser/ForceDeleteCommandParser.java	100.00% <100.00%> (ø)	3.00 <3.00> (?)

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