added "addExpense" mutation

This is a:

[X] :sparkles: New feature - new behaviour has been implemented
[ ] :bug: Bug fix - existing behaviour has been made to behave
[ ] :recycle: Refactor - the behaviour has not changed, just the implementation
[ ] :white_check_mark: Test backfill - tests for existing behaviour were added but the behaviour itself hasn't changed
[ ] :gear: Chore - maintenance task, behaviour and implementation haven't changed

Pull request - read the content of the PR itself (Conversation tab) and consider the following questions:
- Is the user story/ticket linked?
- Does the description match the linked item?
- Does the description tell you how to validate the work?
- Does the build still work (Checks tab)?
- What approach would you take to meet this goal?
Code changes - read through the changeset (Files changed tab) and consider the following questions:
- Do the code changes match the description?
- Are there any changes that should not be part of this pull request (e.g. layout changes in files not related to the functionality)?
- Do you understand what the changes are doing, how they meet the goal of the PR?
- Does the approach to meeting those goals seem sensible? If it's not the one you thought of, is it significantly better or worse?
- Is this well-structured code (consistent layout, comprehensible variable names, relatively small files)?
Software - check out the PR branch, run the software and consider the following questions:
- Does the product still start and run correctly?
- Is the goal of the pull request met (i.e. new behaviour for a new feature, changed behaviour for a bug fix, identical behaviour for a refactor)?
- In the parts of the product this PR touched:
- Is the spelling, punctuation and grammar for user-facing text correct?
- Does the layout/UI match the designs?

tlcoles / BudgetOnTheGo