Software inventory count columns - PR review

PR #300 by Dr4inder · Verdict: merge + fix

Two column headings on Software → Inventory say what they count. Installed On becomes Installations and Seats becomes Licence seats. Nothing else changes: the counts, the sorting (sortBy('install_count'), sortBy('seats')) and the API are exactly as before. User page: Software.

Why it was worth having

Installed On reads as a date - "installed on 3 March" - when the column is a number of machines. The translators read it that way too: German had Installiert am, Greek Εγκαταστάθηκε στις and Spanish Instalado el, all of which mean "installed on (date)". So three languages were showing a heading that described a different thing from the figures under it. Seats was ambiguous beside it; Licence seats says where the number comes from.

What was checked

Question	How	Result
Does anything but the heading change?	Diff of software/index.php	No. Only the two <th> labels; the sortBy() keys, ids and the separate-columns rule (#1549) are untouched
Were existing English strings changed?	Diff of lang/en/software.php	No. New keys col_installations and col_licence_seats; col_installed and col_seats are kept, as the contribution guideline asks - changing existing English leaves every translation of it silently out of date
Does anything else name the old column?	Grepped lang/ for "Installed On"	Yes - the in-app help software.manual_rule_seats said "a nought under Installed On". See below
API or reports affected?	Grep for the keys	No. The API never used the heading text

What was changed

• #2806 - the two new labels in the five complete languages (de, el, es, id, pl), so none of them falls back to English.
• software.manual_rule_seats (the help on manually added applications) rewritten in all six languages: "Licence seats and installations are separate columns. A cloud platform is installed nowhere, so a nought under Installations says nothing about whether anyone uses it".

Merged with git merge --no-ff under the contributor's name (c99167fc6), then the follow-up commit (21aa924fa).

Worth knowing

• col_installed and col_seats are now unused by the page but still in every language file. They are kept on purpose; a later tidy-up can remove them in one go, not one at a time.
• The two columns are separate on purpose (#1549). A cloud application is installed on nothing, so 0 Installations is correct for it; its use is measured by Licence seats. Neither column is a fallback for the other - do not merge them.