Separately store the numbering of iterations

Every iteration should be numbered sequentially and uniquely within a cadence (for auto-generated iterations, the numbering is currently hardcoded in the title).

A newly created iteration gets assigned a number whose value is the highest numbering used in its cadence + 1.

• Make sure to update the documentation with the description of the numbering behavior.

Illustrations:

A set of 3 iterations is created within a cadence (suppose the current date is Oct. 8):

• Iteration: Oct 1 ~ Oct 7 / numbered 1 / CLOSED
• Iteration: Oct 8 ~ Oct 14 / numbered 2 / CURRENT
• Iteration: Oct 15 ~ Oct 21 / numbered 3 / UPCOMING

Case A. Deletes Iteration 3 and a new iteration for Oct 22 ~ Oct 28 is created.

The new iteration gets numbered as 3.

• Iteration: Oct 1 ~ Oct 7 / numbered 1
• Iteration: Oct 8 ~ Oct 14 / numbered 2
• Iteration: Oct 22 ~ Oct 28 / numbered 3

Case B. Deletes Iteration 1 and a new iteration for Oct 1 ~ Oct 7 is created.

The created iteration should get numbered 1.

• Iteration: Oct 1 ~ Oct 7 / numbered 1
• Iteration: Oct 8 ~ Oct 14 / numbered 2
• Iteration: Oct 15 ~ Oct 21 / numbered 3