vendor: add plus icon to fontello

vendor/fontello/font/fontello.svg

@@ -6,6 +6,7 @@
 <font id="fontello" horiz-adv-x="1000" >
 <font-face font-family="fontello" font-weight="400" font-stretch="normal" units-per-em="1000" ascent="850" descent="-150" />
 <missing-glyph horiz-adv-x="1000" />
+<glyph glyph-name="plus" unicode="&#xe801;" d="M0 291v119q0 20 14 35t35 14h281v281q0 20 15 35t35 14h119q20 0 34-14t15-35v-281h281q20 0 35-14t14-35v-119q0-21-14-35t-35-15h-281v-282q0-20-15-34t-34-15h-119q-21 0-35 15t-15 34v282h-281q-20 0-35 15t-14 35z" horiz-adv-x="878.9" />
 <glyph glyph-name="left-open" unicode="&#xe802;" d="M0 353l391 391q18 18 43 18 24 0 43-18 18-18 18-44t-18-43l-304-304 304-305q18-17 18-43t-18-43q-18-18-43-18t-43 18z" horiz-adv-x="495.1" />
 <glyph glyph-name="right-open" unicode="&#xe803;" d="M0 0q0 25 18 43l305 304-305 304q-18 18-18 44t18 43 44 18 43-18l390-391-390-391q-19-18-43-18t-44 18q-18 18-18 44z" horiz-adv-x="495.1" />
 </font>