Move instance setting to logo row

Fixes #145
Remove unneeded method

`adhocracy.lib.search.index.clear` does the same as
`adhocracy.lib.search.drop_all`, but isn't used and thus dropped.
i18n: update German translation
1 d = 86400 s

1 d != 84600 s
Add instance activity to solr index

In order to sort instances by activity, activity information has to be
added to the solr index in the first place.
Update instance information in solr on a daily base (fixes #103)

In order to order instances by activity, activity information has to be
updated regularly. As of how the activity metric works, this has to be
done for all instances at once, and not only if an object in an instance
changes.

For now, this is set up on a daily base.
Merge branch 'develop' into instance-settings-in-logo-row
Fix OpenID after base_url change (fixes #150)

Addendum to ad4c95357faeea8c96f28346fbde2f808162c290
Remove absolute_url in favor of base_url(..., absolute=True)
Replace armlabel against html5 placeholders

Following commit 9ee43dc9c6825cf6671a15213914d6c22a9d6c5a (replace
jquery.label_over with html5 placeholders), the javascript based
armlabel placeholder mechanism is now also replaced by proper html5
placeholders, compatibility assured via the jquery-placeholder polyfill
library.
Add optional `absolute` parameter to h.entity_url
Show absolute path in abuse report email

Another aftermath of base_url change.
Fix URL in import (Closes https://github.com/hhucn/adhocracy.hhu_theme/issues/74)
Merge branch 'develop' of github.com:liqd/adhocracy into develop
shortlinks should be absolute (Fixes https://github.com/hhucn/adhocracy.hhu_theme/issues/73)
Use markdown safe_mode where appropriate

* Add safe_mode keyword argument to render and markdown_to_plain_text
functions
* Disable attributes
* Do not CGI escape
* Use xhtml style html5 output format
Don't show raw html in event short text
i18n: update German translation
Use base_url instead of absolute_url

Addendum to 2822a7aa900d43c0fe318ee9d134a42f4c96c76f
Improve available number of pager items in listings (fixes #151)

* Use the 10, 20, 50, 100, 200 ... sequence scheme for available listing
numbers.
* Increase maximum listing from 250 to 500
* Don't show the currently selected option as a link
* Change available options in user selection