ActivityCommand substring bugfix and contact command prevent duplicate name

[daekoon]

Context

There was an edge case that happened when there were 2 contacts, with one contact's name being the substring of another.

Example:
Contact 1: George
Contact 2: George Yeoh

You will never be able to add Contact 1 in because when you search "George", it will always return 2 matches regardless.

Also, there is no validation to check if two person have the same name.

Changes

• Added additional case for ActivityCommand, which now first checks if there is an exact match with the name. It will proceed to keyword based searching if there isn't.
• Added validation for Contact command, to check for duplicate names
• Added relevant tests
• Fixed broken ActivityCommand tests
• Update UserGuide and Developer Guide

[liakify]

looks good to me, at some point we should centralise the name searching logic into a single place cause currently activity, expense, invite, disinvite supposed to use the same thing

after merging this then expense, invite, disinvite will encounter the same issue