Add weekly investment plan

[mierin12]

Closes #1396
Closes #4330
Issue https://forum.portfolio-performance.info/t/intervall-von-sparplaenen/5655

Hello, this is a proposition to add weekly (and bi-weekly) intervals to investment plan.
What I call "bi-weekly" is every two weeks = every two Mondays/Tuesdays etc. This is different than twice a month.

I think "twice a month" is also a wished interval but it seems more complicated to do, and a workaround with two monthly plans is already possible. But there was no workaround for weekly or bi weekly intervals.

When an "each Monday" date is an holiday, the transaction is made on the next non-holiday date, and the following transactions are put back to "each Monday".
If the start date itself is on a holiday Monday, the first start date is offset in the same way, and the next transactions are kept on Mondays.

A test is added in InvestmentPlanTest.

I have also changed the items shown in the column Interval when you want to switch the interval :
Before :

After :

Internally, I used 100 and 200 for the weekly and biweekly interval integers since I believe there will never be an "every 100 months" interval. It is very easy to add an "every three weeks" interval if relevant.