Add tour book statistics

[chrgernoe]

First if all, this issue is a general feature request for tour book statistics but should also be a place to discuss reasonable statistical features. However, each of the agreed statistical features should be described in a separate issue.

Ideas:

• Grades climbed in general
• Grades climbed over time
• Rocks missing in a certain region
• New rocks ascended during a certain period

Sub-tasks:

• Add empty statistics view #243
• Number of ascends per year #244
• Most frequent climbing partners #439
• Number of ascents per climbed grade #461
• Number of newly collected rocks per year #464

[paetz]

The different statistics should be implemented independently to keep PRs small.