Reduce code duplication for common fields in expiring cmd, refactor msg

[sherrpass]

1. Updated command messages and UG to include expired policies
2. Decrease duplicated constants between showexpiringpolicies and showexpiringpoliciessummary commands

[codecov-commenter]

Codecov Report

Merging #185 (bfa2bbc) into master (dfbc77d) will decrease coverage by 0.23%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master     #185      +/-   ##
============================================
- Coverage     54.84%   54.61%   -0.24%     
- Complexity      556      562       +6     
============================================
  Files           107      107              
  Lines          2281     2298      +17     
  Branches        282      287       +5     
============================================
+ Hits           1251     1255       +4     
- Misses          933      944      +11     
- Partials         97       99       +2     

Impacted Files	Coverage Δ
...mands/policy/ShowExpiringPolicySummaryCommand.java	42.85% <0.00%> (-3.81%)	⬇️
...gic/commands/policy/ShowExpiringPolicyCommand.java	37.50% <100.00%> (ø)
...ava/seedu/siasa/model/policy/UniquePolicyList.java	71.42% <0.00%> (-2.65%)	⬇️
...va/seedu/siasa/logic/commands/DownloadCommand.java	1.96% <0.00%> (-0.17%)	⬇️
...in/java/seedu/siasa/commons/util/CurrencyUtil.java	0.00% <0.00%> (ø)

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[yatbun]

Standardising of output messages

[yatbun]

LGTM