Fix wiki search overflowing on wide screens (#6047)

Confine the search menu to be at most the width of the page, or 80% of
the viewport width, whichever is smaller. To do this, introduce a new
`--container-width` variable for the descendant elements of
`.ui.container` to be able to access.

Also update the relevant e2e test: add a long 'lorem ipsum' page, add a
search for it, parameterize the width.

tests/gitea-repositories-meta/user2/repo1.wiki.git/objects/d4/9ac742d44063dcf69d4e0afe725813b777dd89

@@ -0,0 +1 @@
+x
ťŹÍn›@…»ž§¸ű*ń†áGjŞŚmJ±‰C1–“ě~R0ŹÓ§i}’*ďU·•şĘ*gw>éHçËş¶­5<C2AD>6}§Ą€3”)Sh˛´0—9ÍTƨáž;2i2Óĺ®-É^j§
óLň4wSžŰŞp™mSTČ­‚Ű\9V†Ô1Sép"Źşę¸ŐĽ|ůůëÇ“„ťî.Z>É법us™uíG 63<36>…Ąđ)"9ÓóG­Ţ¶.÷ĺˇ.áâO¦ž¬ ň#XţJ$›ŘűË	¨?o<1˝_Š)n<>/±~ćá6ąëŐcS­űl<ÔXŽiŇ–b3{@ź÷B,g±řĎ	”Ż‘ŰËK|˝ĆEĐ[ß0\ď<>ÉŹaWńݢRć|¤ři{şąžxd…ŤĂÉßô
9<ĐŮŇŹšg>»¨ŠKOvÉ"D§J„sEŕęŘśzňĎĆ[Í_s!2ϡév%ěe©Čo(˝”

tests/gitea-repositories-meta/user2/repo1.wiki.git/refs/heads/master

@@ -1 +1 @@
-0dca5bd9b5d7ef937710e056f575e86c0184ba85
+d49ac742d44063dcf69d4e0afe725813b777dd89