Various improvements

[frankrousseau]

Problem

• Budget exceptions cannot be enforced.
• Budgets can't be exported to .csv.
• Drawings data can't be imported/exported.

Solution

• Allow modification of a monthly salary at the entry level.
• Allow modification of a monthly salary during a given month (use the exception field added on the backend side).
• Add an export button that generates the budget .csv file.

[NicoPennec]

a <span> is mandatory?

To reduce DOM you can use <template>:

Suggested change

	<span v-else>&nbsp;</span>
	<template v-else>&nbsp;</template>