Allow Assign command to add persons to weddings

[yikjunxian]

Examples:

• assign INDEX/NAME r/[ROLE]
• assign INDEX/NAME w/[WEDDING INDEX]
• assign INDEX/NAME r/[ROLE] w/[WEDDING INDEX 1] w/[WEDDING INDEX 2]

Notes:

• will fix broken tests asap
• not quite sure how weddingJobs should be saved in Json file
• should Weddings have an attribute with the list of persons that are involved in that wedding (?)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 82.97872% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...va/seedu/address/logic/commands/AssignCommand.java	76.27%	5 Missing and 9 partials :warning:
.../seedu/address/logic/parser/AddressBookParser.java	0.00%	1 Missing :warning:
...eedu/address/logic/parser/AssignCommandParser.java	96.77%	0 Missing and 1 partial :warning:

Files with missing lines	Coverage Δ	Complexity Δ
src/main/java/seedu/address/logic/Messages.java	92.85% <100.00%> (+0.85%)	5.00 <1.00> (+1.00)
.../seedu/address/logic/parser/AddressBookParser.java	76.92% <0.00%> (ø)	13.00 <0.00> (ø)
...eedu/address/logic/parser/AssignCommandParser.java	96.77% <96.77%> (ø)	11.00 <11.00> (?)
...va/seedu/address/logic/commands/AssignCommand.java	69.76% <76.27%> (ø)	22.00 <7.00> (?)